Precalculus Quiz: Proving The Pythagorean Identity

What this quiz covers

This quiz focuses on Proving The Pythagorean Identity, giving you a quick way to practice the rules, question types, and explanations that matter most for Precalculus.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. $\theta = \frac{\pi}{4}$, where $\sin(\frac{\pi}{4}) + \cos(\frac{\pi}{4}) = \frac{\sqrt{2}}{2} + \frac{\sqrt{2}}{2} = \sqrt{2} \neq 1$ (correct answer)
2. $\theta = \frac{\pi}{6}$, where $\sin(\frac{\pi}{6}) + \cos(\frac{\pi}{6}) = \frac{1}{2} + \frac{\sqrt{3}}{2} = \frac{1 + \sqrt{3}}{2} \neq 1$
3. $\theta = \frac{\pi}{3}$, where $\sin(\frac{\pi}{3}) + \cos(\frac{\pi}{3}) = \frac{\sqrt{3}}{2} + \frac{1}{2} = \frac{1 + \sqrt{3}}{2} \neq 1$
4. $\theta = \pi$, where $\sin(\pi) + \cos(\pi) = 0 + (-1) = -1 \neq 1$

Explanation: The student's error is assuming that $\sqrt{a^2 + b^2} = a + b$, which is false. To find the best counterexample, we want a case where the difference between $\sin(\theta) + \cos(\theta)$ and 1 is most obvious. For $\theta = \frac{\pi}{4}$: $\sin(\frac{\pi}{4}) + \cos(\frac{\pi}{4}) = \frac{\sqrt{2}}{2} + \frac{\sqrt{2}}{2} = \sqrt{2} \approx 1.414$. This gives the clearest counterexample because $\sqrt{2}$ is a well-known irrational number distinctly different from 1. Choices B and C give $\frac{1 + \sqrt{3}}{2} \approx 1.366$, which is also not 1, but the calculation is more complex. Choice D gives -1, which while clearly ≠ 1, uses a less intuitive angle for demonstrating the fundamental algebraic error.

Question 2

1. $-\frac{4}{5} + \frac{3}{4}$
2. $-\frac{4}{5} - \frac{3}{4}$ (correct answer)
3. $\frac{4}{5} - \frac{3}{4}$
4. $\frac{4}{5} + \frac{3}{4}$

Explanation: Using the Pythagorean identity: $\sin^2(\theta) + \cos^2(\theta) = 1$, so $\cos^2(\theta) = 1 - (\frac{3}{5})^2 = 1 - \frac{9}{25} = \frac{16}{25}$. Therefore $\cos(\theta) = \pm\frac{4}{5}$. Since $\theta$ is in Quadrant II, cosine is negative, so $\cos(\theta) = -\frac{4}{5}$. Then $\tan(\theta) = \frac{\sin(\theta)}{\cos(\theta)} = \frac{\frac{3}{5}}{-\frac{4}{5}} = -\frac{3}{4}$. Thus $\cos(\theta) + \tan(\theta) = -\frac{4}{5} + (-\frac{3}{4}) = -\frac{4}{5} - \frac{3}{4}$. Choice A incorrectly makes tangent positive. Choice C incorrectly makes cosine positive. Choice D incorrectly makes both cosine and tangent positive.

Question 3

1. $-\frac{12}{13}$ (correct answer)
2. $\frac{12}{13}$
3. $\pm\frac{12}{13}$
4. $-\frac{5}{13}$

Explanation: This question tests understanding of the Pythagorean identity sin²(θ) + cos²(θ) = 1 and how to use it to find missing trig values. The Pythagorean identity states that sin²(θ) + cos²(θ) = 1 for any angle θ, which means that if you know one of these trig functions, you can find the other using the rearranged form sin²(θ) = 1 - cos²(θ) or cos²(θ) = 1 - sin²(θ). Given cos(θ) = 5/13, we rearrange the identity to sin²(θ) = 1 - cos²(θ) = 1 - (5/13)² = 1 - 25/169 = 144/169, so sin(θ) = ±√(144/169) = ±12/13. The quadrant information tells us sine is negative in Quadrant III, giving sin(θ) = -12/13. Choice A is correct because it properly applies the identity with correct arithmetic and uses the right quadrant to determine the sign. Choice B uses the wrong sign for sine, forgetting that in Quadrant III, sine is negative. Key to using the Pythagorean identity: when given one trig value (sin or cos), use the identity to isolate the unknown, then take the square root and determine the correct sign based on which quadrant the angle is in. Remember the quadrant sign rules: Quadrant I (both positive), Quadrant II (sin positive, cos negative), Quadrant III (both negative), Quadrant IV (sin negative, cos positive)—use these to choose the correct sign after taking the square root.

Question 4

1. $-\frac{4}{5}$ (correct answer)
2. $\frac{4}{5}$
3. $-\frac{3}{5}$
4. $\pm\frac{4}{5}$

Explanation: This question tests understanding of the Pythagorean identity sin²(θ) + cos²(θ) = 1 and how to use it to find missing trig values when given tan(θ). The Pythagorean identity states that sin²(θ) + cos²(θ) = 1 for any angle θ, and combined with tan(θ) = sin(θ)/cos(θ), we can find both sin(θ) and cos(θ). Given tan(θ) = 4/3 and θ in Quadrant III, we know sin(θ)/cos(θ) = 4/3, so sin(θ) = (4/3)cos(θ). Substituting into sin²(θ) + cos²(θ) = 1 gives (16/9)cos²(θ) + cos²(θ) = 1, which simplifies to (25/9)cos²(θ) = 1, so cos²(θ) = 9/25 and cos(θ) = -3/5 (negative in Quadrant III). Therefore, sin(θ) = (4/3)(-3/5) = -4/5. Choice A is correct because it properly combines the Pythagorean identity with the tangent relationship and correctly determines that both sine and cosine are negative in Quadrant III. Choice B uses the wrong sign for sine, forgetting that in Quadrant III, sine is negative. Key to using the Pythagorean identity: when given one trig value (sin or cos), use the identity to find the other by rearranging to isolate the unknown, then take the square root and determine the correct sign based on which quadrant the angle is in.

Question 5

1. $\frac{1}{4}$
2. $\frac{3}{4}$ (correct answer)
3. $-\frac{3}{4}$
4. $\frac{1}{2}$

Explanation: This question tests understanding of the Pythagorean identity sin²(θ) + cos²(θ) = 1 and how to use it to find missing trig values. The Pythagorean identity states that sin²(θ) + cos²(θ) = 1 for any angle θ, which means that if you know one of these trig functions, you can find the other using the rearranged form sin²(θ) = 1 - cos²(θ) or cos²(θ) = 1 - sin²(θ). Given sin(θ) = 1/2, we use the identity to find cos²(θ) = 1 - sin²(θ) = 1 - (1/2)² = 1 - 1/4 = 3/4. Since the question asks for cos²(θ), no square root or sign determination is needed. Choice B is correct because it properly applies the identity with correct arithmetic. Choice C incorrectly adds a negative sign, but since it's cos², it should be positive. Key to using the Pythagorean identity: when given one trig value (sin or cos), use the identity to rearrange to isolate the unknown, then take the square root and determine the correct sign based on which quadrant the angle is in. The Pythagorean identity sin²(θ) + cos²(θ) = 1 is one of the most fundamental trig identities: it works for any angle, derives directly from either the unit circle or the Pythagorean theorem, and is essential for solving countless trig problems.

Question 6

1. The expression equals $(\sin^2(x) + \cos^2(x))^2$, and since $\sin^2(x) + \cos^2(x) = 1$, the result is $1^2 = 1$ (correct answer)
2. The expression can be factored as $\sin^2(x)(\sin^2(x) + 2\cos^2(x)) + \cos^4(x) = 1$ using the identity
3. Since $\sin^4(x) = (\sin^2(x))^2$ and $\cos^4(x) = (\cos^2(x))^2$, we apply the identity to each term separately
4. The expression simplifies by substituting $\cos^2(x) = 1 - \sin^2(x)$ into each term and then expanding completely

Explanation: The key insight is recognizing that $\sin^4(x) + \cos^4(x) + 2\sin^2(x)\cos^2(x)$ is a perfect square: $(\sin^2(x) + \cos^2(x))^2 = (\sin^2(x))^2 + 2\sin^2(x)\cos^2(x) + (\cos^2(x))^2 = \sin^4(x) + 2\sin^2(x)\cos^2(x) + \cos^4(x)$. By the Pythagorean identity, $\sin^2(x) + \cos^2(x) = 1$, so the expression equals $1^2 = 1$. Choice B shows an incorrect factorization. Choice C incorrectly suggests applying the identity to individual terms. Choice D suggests a more complicated substitution method that, while possible, doesn't reveal the elegant structure.

Question 7

1. Substitute $x=\cos(\theta)$ and $y=\sin(\theta)$ into $x^2+y^2=1$ to get $\cos^2(\theta)+\sin^2(\theta)=1$. (correct answer)
2. Substitute $x=\sin(\theta)$ and $y=\cos(\theta)$ into $x^2+y^2=1$ to get $\sin(\theta)+\cos(\theta)=1$.
3. Differentiate $x^2+y^2=1$ with respect to $\theta$ to get $\sin^2(\theta)+\cos^2(\theta)=1$.
4. Square both sides of $\sin(\theta)+\cos(\theta)=1$ to obtain $\sin^2(\theta)+\cos^2(\theta)=1$.

Explanation: This question tests understanding of the Pythagorean identity sin²(θ) + cos²(θ) = 1 and how it derives from the unit circle. The Pythagorean identity derives from the unit circle equation x² + y² = 1: since a point at angle θ on the unit circle has coordinates (cos(θ), sin(θ)), substituting gives cos²(θ) + sin²(θ) = 1. On the unit circle with radius 1, any point satisfies x² + y² = 1. Since the coordinates at angle θ are (cos(θ), sin(θ)), substituting x = cos(θ) and y = sin(θ) into the circle equation gives cos²(θ) + sin²(θ) = 1. Choice A is correct because it correctly describes the unit circle derivation. Choice B confuses the coordinates, using (sin(θ), cos(θ)) instead of (cos(θ), sin(θ)) on the unit circle. The Pythagorean identity sin²(θ) + cos²(θ) = 1 is one of the most fundamental trig identities: it works for any angle, derives directly from either the unit circle or the Pythagorean theorem, and is essential for solving countless trig problems. Don't confuse the Pythagorean identity with similar-looking statements: sin(θ) + cos(θ) does NOT equal 1 (missing squares), and sin²(θ) + cos²(θ) always equals 1, not 0 or any other number.

Question 8

1. $\sin(\theta)+\cos(\theta)=1$
2. $\sin^2(\theta)-\cos^2(\theta)=1$
3. $\sin^2(\theta)+\cos^2(\theta)=1$ (correct answer)
4. $\sin^2(\theta)+\cos(\theta)=1$

Explanation: This question tests understanding of how the Pythagorean identity derives from the unit circle. The Pythagorean identity derives from the unit circle equation x² + y² = 1: since a point at angle θ on the unit circle has coordinates (cos(θ), sin(θ)), substituting gives cos²(θ) + sin²(θ) = 1. On the unit circle with radius 1, any point satisfies x² + y² = 1. Since the coordinates at angle θ are (cos(θ), sin(θ)), substituting x = cos(θ) and y = sin(θ) into the circle equation gives cos²(θ) + sin²(θ) = 1. Choice C is correct because it correctly describes the unit circle derivation with the proper squared terms. Choice A omits the squares in the identity, incorrectly stating sin(θ) + cos(θ) = 1, when the correct identity requires sin²(θ) + cos²(θ) = 1. Don't confuse the Pythagorean identity with similar-looking statements: sin(θ) + cos(θ) does NOT equal 1 (missing squares), and sin²(θ) + cos²(θ) always equals 1, not 0 or any other number.

Question 9

1. $\sin^2(\phi) = 1 - \frac{4}{9} = \frac{5}{9}$, so $\sin(\phi) = \frac{\sqrt{5}}{3}$
2. $\sin^2(\phi) = 1 - \frac{4}{9} = \frac{5}{9}$, so $\sin(\phi) = -\frac{\sqrt{5}}{3}$ (correct answer)
3. $\sin^2(\phi) = 1 + \frac{4}{9} = \frac{13}{9}$, so $\sin(\phi) = -\frac{\sqrt{13}}{3}$
4. $\sin^2(\phi) = \frac{4}{9} - 1 = -\frac{5}{9}$, so $\sin(\phi) = -\frac{\sqrt{5}}{3}$

Explanation: The Pythagorean identity states $\sin^2(\phi) + \cos^2(\phi) = 1$. Solving for $\sin^2(\phi)$: $\sin^2(\phi) = 1 - \cos^2(\phi) = 1 - (\frac{2}{3})^2 = 1 - \frac{4}{9} = \frac{5}{9}$. Since $\phi$ is in Quadrant IV, sine is negative, so $\sin(\phi) = -\sqrt{\frac{5}{9}} = -\frac{\sqrt{5}}{3}$. Choice A has the correct calculation but wrong sign. Choice C incorrectly adds instead of subtracting $\cos^2(\phi)$. Choice D incorrectly rearranges the identity and gets a negative value under the square root.

Question 10

1. $\frac{9}{25}$
2. $\frac{16}{25}$ (correct answer)
3. $\frac{4}{5}$
4. $\frac{1}{25}$

Explanation: This question tests understanding of the Pythagorean identity sin²(θ) + cos²(θ) = 1 and how to use it to find missing trig values. The Pythagorean identity states that sin²(θ) + cos²(θ) = 1 for any angle θ, which means that if you know one of these trig functions, you can find the other using the rearranged form sin²(θ) = 1 - cos²(θ) or cos²(θ) = 1 - sin²(θ). Given sin(θ) = 3/5, we use the identity to find cos²(θ) = 1 - sin²(θ) = 1 - (3/5)² = 1 - 9/25 = 16/25. Choice B is correct because it properly applies the identity with correct arithmetic. Choice A makes an arithmetic error, calculating 9/25 instead of 16/25. Key to using the Pythagorean identity: when given one trig value (sin or cos), use the identity to find the other by rearranging to isolate the unknown, then take the square root and determine the correct sign based on which quadrant the angle is in. The Pythagorean identity sin²(θ) + cos²(θ) = 1 is one of the most fundamental trig identities: it works for any angle, derives directly from either the unit circle or the Pythagorean theorem, and is essential for solving countless trig problems.

Question 11

1. Divide $a^2+b^2=c^2$ by $c$ to get $\left(\frac{a}{c}\right)^2+\left(\frac{b}{c}\right)^2=c$.
2. Divide $a^2+b^2=c^2$ by $c^2$ to get $\left(\frac{a}{c}\right)^2+\left(\frac{b}{c}\right)^2=1$, then use $\sin(\theta)=\frac{a}{c}$ and $\cos(\theta)=\frac{b}{c}$. (correct answer)
3. Use $a^2+b^2=c^2$ and substitute $\sin(\theta)=\frac{c}{a}$ and $\cos(\theta)=\frac{c}{b}$.
4. Add $\sin(\theta)$ and $\cos(\theta)$ to get $\sin(\theta)+\cos(\theta)=1$.

Explanation: This question tests understanding of the Pythagorean identity sin²(θ) + cos²(θ) = 1 and how it derives from right triangles. The Pythagorean identity comes from the Pythagorean theorem in a right triangle: starting with a² + b² = c² and dividing both sides by c², we get (a/c)² + (b/c)² = 1, which becomes sin²(θ) + cos²(θ) = 1 since sin(θ) = a/c and cos(θ) = b/c. In a right triangle with opposite side a, adjacent side b, and hypotenuse c, the Pythagorean theorem gives a² + b² = c². Dividing every term by c² yields (a/c)² + (b/c)² = 1. Since sin(θ) = a/c (opposite/hypotenuse) and cos(θ) = b/c (adjacent/hypotenuse), this becomes sin²(θ) + cos²(θ) = 1. Choice B is correct because it accurately states the identity. Choice A incorrectly derives the identity, failing to divide by c² in the Pythagorean theorem, leaving a² + b² = c² instead of the ratio form. The Pythagorean identity sin²(θ) + cos²(θ) = 1 is one of the most fundamental trig identities: it works for any angle, derives directly from either the unit circle or the Pythagorean theorem, and is essential for solving countless trig problems. Don't confuse the Pythagorean identity with similar-looking statements: sin(θ) + cos(θ) does NOT equal 1 (missing squares), and sin²(θ) + cos²(θ) always equals 1, not 0 or any other number.

Question 12

1. Substitute $x=\sin(\theta)$ and $y=\cos(\theta)$ into $x^2+y^2=2$.
2. Substitute $x=\cos(\theta)$ and $y=\sin(\theta)$ into $x^2+y^2=1$ to get $\cos^2(\theta)+\sin^2(\theta)=1$. (correct answer)
3. Use $x+y=1$ and set $x=\cos(\theta)$, $y=\sin(\theta)$ to get $\sin(\theta)+\cos(\theta)=1$.
4. Differentiate $x^2+y^2=1$ to obtain $\sin^2(\theta)+\cos^2(\theta)=1$.

Explanation: This question tests understanding of the Pythagorean identity sin²(θ) + cos²(θ) = 1 and how it derives from the unit circle. The Pythagorean identity derives from the unit circle equation x² + y² = 1: since a point at angle θ on the unit circle has coordinates (cos(θ), sin(θ)), substituting gives cos²(θ) + sin²(θ) = 1. On the unit circle with radius 1, any point satisfies x² + y² = 1. Since the coordinates at angle θ are (cos(θ), sin(θ)), substituting x = cos(θ) and y = sin(θ) into the circle equation gives cos²(θ) + sin²(θ) = 1. Choice B is correct because it correctly describes the unit circle derivation. Choice A uses the wrong equation x² + y² = 2 instead of 1. The Pythagorean identity sin²(θ) + cos²(θ) = 1 is one of the most fundamental trig identities: it works for any angle, derives directly from either the unit circle or the Pythagorean theorem, and is essential for solving countless trig problems. Don't confuse the Pythagorean identity with similar-looking statements: sin(θ) + cos(θ) does NOT equal 1 (missing squares), and sin²(θ) + cos²(θ) always equals 1, not 0 or any other number.

Question 13

1. $\frac{4}{5}$
2. $-\frac{4}{5}$ (correct answer)
3. $-\frac{3}{5}$
4. $\pm\frac{4}{5}$

Explanation: This question tests understanding of the Pythagorean identity sin²(θ) + cos²(θ) = 1 and how to use it to find missing trig values. The Pythagorean identity states that sin²(θ) + cos²(θ) = 1 for any angle θ, which means that if you know one of these trig functions, you can find the other using the rearranged form sin²(θ) = 1 - cos²(θ) or cos²(θ) = 1 - sin²(θ). Given sin(θ) = 3/5, we use the identity to find cos²(θ) = 1 - sin²(θ) = 1 - (3/5)² = 1 - 9/25 = 16/25. Taking the square root gives cos(θ) = ±√(16/25) = ±4/5, and since θ is in Quadrant II, where cosine is negative, we choose cos(θ) = -4/5. Choice B is correct because it properly applies the identity with correct arithmetic and uses the right quadrant to determine the sign. Choice A uses the wrong sign for cosine, forgetting that in Quadrant II, cosine is negative. Key to using the Pythagorean identity: when given one trig value (sin or cos), use the identity to isolate the unknown, then take the square root and determine the correct sign based on which quadrant the angle is in. Remember the quadrant sign rules: Quadrant I (both positive), Quadrant II (sin positive, cos negative), Quadrant III (both negative), Quadrant IV (sin negative, cos positive)—use these to choose the correct sign after taking the square root.

Question 14

1. $\frac{\sqrt{5}}{3}$ (correct answer)
2. $-\frac{\sqrt{5}}{3}$
3. $\frac{5}{9}$
4. $\pm\frac{\sqrt{5}}{3}$

Explanation: This question tests understanding of the Pythagorean identity sin²(θ) + cos²(θ) = 1 and how to use it to find missing trig values. The Pythagorean identity states that sin²(θ) + cos²(θ) = 1 for any angle θ, which means that if you know one of these trig functions, you can find the other using the rearranged form sin²(θ) = 1 - cos²(θ) or cos²(θ) = 1 - sin²(θ). Given cos(θ) = 2/3, we rearrange the identity to sin²(θ) = 1 - cos²(θ) = 1 - (2/3)² = 1 - 4/9 = 5/9, so sin(θ) = ±√(5/9) = ±√5/3. The quadrant information tells us sine is positive in Quadrant I (acute angle), giving sin(θ) = √5/3. Choice A is correct because it properly applies the identity and correctly uses the positive sign for sine in Quadrant I. Choice C forgets to take the square root after finding sin²(θ) = 5/9, giving the squared value instead of sin(θ) = √5/3. Key to using the Pythagorean identity: when given one trig value (sin or cos), use the identity to find the other by rearranging to isolate the unknown, then take the square root and determine the correct sign based on which quadrant the angle is in.

Question 15

1. Finding that $\sin(\beta) = \frac{7}{25}$ and $\cos(\beta) = \frac{24}{25}$ from the tangent ratio
2. Verifying that $\frac{7}{24}$ represents the ratio of opposite to adjacent sides
3. Determining that the hypotenuse length is $25$ when opposite is $7$ and adjacent is $24$ (correct answer)
4. Confirming that both sine and cosine values are positive in Quadrant I

Explanation: The Pythagorean identity $\sin^2(\beta) + \cos^2(\beta) = 1$ is equivalent to the Pythagorean theorem $a^2 + b^2 = c^2$ in a right triangle. To find the hypotenuse when we know opposite = 7 and adjacent = 24, we use $7^2 + 24^2 = c^2$, giving $49 + 576 = 625$, so $c = 25$. This directly applies the Pythagorean identity. Choice A uses the results after applying the identity. Choice B is just understanding the definition of tangent. Choice D involves quadrant analysis, not the Pythagorean identity.

Question 16

1. $-\frac{288}{169}$ (correct answer)
2. $-\frac{144}{169}$
3. $\frac{144}{169}$
4. $\frac{288}{169}$

Explanation: From the Pythagorean identity: $\sin^2(\alpha) = 1 - \cos^2(\alpha) = 1 - \frac{25}{169} = \frac{144}{169}$. Since $\sin(\alpha) > 0$, we have $\sin(\alpha) = \frac{12}{13}$. Then $\tan(\alpha) = \frac{\sin(\alpha)}{\cos(\alpha)} = \frac{\frac{12}{13}}{-\frac{5}{13}} = -\frac{12}{5}$, so $\tan^2(\alpha) = \frac{144}{25}$. Therefore $\sin^2(\alpha) - \tan^2(\alpha) = \frac{144}{169} - \frac{144}{25} = \frac{144 \cdot 25 - 144 \cdot 169}{169 \cdot 25} = \frac{144(25-169)}{4225} = \frac{144(-144)}{4225} = -\frac{288}{169}$. Choice B uses an incorrect calculation. Choice C has the wrong sign. Choice D has both wrong calculation and wrong sign.

Question 17

1. $\frac{3}{5}$
2. $-\frac{3}{5}$ (correct answer)
3. $\frac{4}{5}$
4. $-\frac{4}{5}$

Explanation: This question tests understanding of the Pythagorean identity $\sin^2(\theta) + \cos^2(\theta) = 1$ and how to use it to find missing trig values. The Pythagorean identity states that $\sin^2(\theta) + \cos^2(\theta) = 1$ for any angle θ, which means that if you know one of these trig functions, you can find the other using the rearranged form $\sin^2(\theta) = 1 - \cos^2(\theta)$ or $\cos^2(\theta) = 1 - \sin^2(\theta)$. Given $\tan(\theta) = \frac{4}{3}$, we can use a right triangle where opposite = 4, adjacent = 3, hypotenuse = 5, so $\cos(\theta) = \frac{\text{adjacent}}{\text{hypotenuse}} = \frac{3}{5}$ in magnitude; since θ is in Quadrant III, where cosine is negative, $\cos(\theta) = -\frac{3}{5}$. Choice B is correct because it properly applies the identity with correct arithmetic and uses the right quadrant to determine the negative sign. Choice A uses the wrong sign for cosine, forgetting that in Quadrant III, cosine is negative. Key to using the Pythagorean identity: when given one trig value (sin or cos), use the identity to find the other by rearranging to isolate the unknown, then take the square root and determine the correct sign based on which quadrant the angle is in. Remember the quadrant sign rules: Quadrant I (both positive), Quadrant II (sin positive, cos negative), Quadrant III (both negative), Quadrant IV (sin negative, cos positive)—use these to choose the correct sign after taking the square root.

Question 18

1. $\frac{12}{13}$
2. $-\frac{12}{13}$ (correct answer)
3. $-\frac{5}{13}$
4. $\pm\frac{12}{13}$

Explanation: This question tests understanding of the Pythagorean identity sin²(θ) + cos²(θ) = 1 and how to use it to find missing trig values. The Pythagorean identity states that sin²(θ) + cos²(θ) = 1 for any angle θ, which means that if you know one of these trig functions, you can find the other using the rearranged form sin²(θ) = 1 - cos²(θ) or cos²(θ) = 1 - sin²(θ). Given cos(θ) = 5/13, we rearrange the identity to sin²(θ) = 1 - cos²(θ) = 1 - (5/13)² = 1 - 25/169 = 144/169, so sin(θ) = ±√(144/169) = ±12/13. The quadrant information tells us sine is negative in Quadrant IV, giving sin(θ) = -12/13. Choice B is correct because it properly applies the identity and correctly determines that sine is negative in Quadrant IV. Choice A uses the wrong sign for sine, forgetting that in Quadrant IV, sine is negative. Key to using the Pythagorean identity: when given one trig value (sin or cos), use the identity to find the other by rearranging to isolate the unknown, then take the square root and determine the correct sign based on which quadrant the angle is in.

Question 19

1. $\frac{3}{5}$ (correct answer)
2. $-\frac{3}{5}$
3. $\pm\frac{3}{5}$
4. $\frac{4}{5}$

Explanation: This question tests understanding of the Pythagorean identity $\sin^2(\theta) + \cos^2(\theta) = 1$ and how to use it to find missing trig values. The Pythagorean identity states that $\sin^2(\theta) + \cos^2(\theta) = 1$ for any angle $\theta$, which means that if you know one of these trig functions, you can find the other using the rearranged form $\sin^2(\theta) = 1 - \cos^2(\theta)$ or $\cos^2(\theta) = 1 - \sin^2(\theta)$. Given $\cos(\theta) = \frac{4}{5}$, we rearrange the identity to $\sin^2(\theta) = 1 - \cos^2(\theta) = 1 - \left( \frac{4}{5} \right)^2 = 1 - \frac{16}{25} = \frac{9}{25}$, so $\sin(\theta) = \pm \sqrt{\frac{9}{25}} = \pm \frac{3}{5}$. The quadrant information tells us sine is positive in Quadrant I, giving $\sin(\theta) = \frac{3}{5}$. Choice A is correct because it properly applies the identity with correct arithmetic and uses the right quadrant to determine the positive sign. Choice C provides both $\pm$ solutions when the quadrant information specifies a unique sign. Key to using the Pythagorean identity: when given one trig value (sin or cos), use the identity to find the other by rearranging to isolate the unknown, then take the square root and determine the correct sign based on which quadrant the angle is in. Remember the quadrant sign rules: Quadrant I (both positive), Quadrant II (sin positive, cos negative), Quadrant III (both negative), Quadrant IV (sin negative, cos positive)—use these to choose the correct sign after taking the square root.

Question 20

1. $\frac{\sqrt{5}}{3}$ (correct answer)
2. $-\frac{\sqrt{5}}{3}$
3. $\frac{5}{9}$
4. $\pm\frac{\sqrt{5}}{3}$

Explanation: This question tests understanding of the Pythagorean identity $\sin^2(\theta) + \cos^2(\theta) = 1$ and how to use it to find missing trig values. The Pythagorean identity states that $\sin^2(\theta) + \cos^2(\theta) = 1$ for any angle $\theta$, which means that if you know one of these trig functions, you can find the other using the rearranged form $\sin^2(\theta) = 1 - \cos^2(\theta)$ or $\cos^2(\theta) = 1 - \sin^2(\theta)$. Given $\cos(\theta) = \frac{2}{3}$, we rearrange the identity to $\sin^2(\theta) = 1 - \cos^2(\theta) = 1 - \left(\frac{2}{3}\right)^2 = 1 - \frac{4}{9} = \frac{5}{9}$, so $\sin(\theta) = \pm \sqrt{\frac{5}{9}} = \pm \frac{\sqrt{5}}{3}$. The quadrant information tells us sine is positive, giving $\sin(\theta) = \frac{\sqrt{5}}{3}$. Choice A is correct because it properly applies the identity with correct arithmetic and uses the right quadrant to determine sign. Choice B uses the wrong sign for sine, forgetting that for an acute angle, sine is positive. Key to using the Pythagorean identity: when given one trig value (sin or cos), use the identity to rearrange to isolate the unknown, then take the square root and determine the correct sign based on which quadrant the angle is in. Remember the quadrant sign rules: Quadrant I (both positive), Quadrant II (sin positive, cos negative), Quadrant III (both negative), Quadrant IV (sin negative, cos positive)—use these to choose the correct sign after taking the square root.