Using Conjugates with Complex Numbers

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Algebra 2 › Using Conjugates with Complex Numbers

Questions 1 - 10
1

Use the conjugate method to simplify and write in $a+bi$ form: $$\frac{2-i}{3+2i}.$$

$\frac{4}{13} - \frac{7}{13}i$

$\frac{4}{13} + \frac{7}{13}i$

$\frac{2-i}{3-2i}$

$\frac{8}{13} - \frac{1}{13}i$

Explanation

This question tests your understanding of complex conjugates—pairs of complex numbers that differ only in the sign of their imaginary part—and using them to rationalize denominators. The conjugate of a + bi is a - bi; multiply numerator and denominator by it to eliminate i below. For (2 - i)/(3 + 2i), conjugate of denominator 3 - 2i; numerator (2 - i)(3 - 2i)=23=6, 2(-2i)=-4i, -i3=-3i, -i(-2i)=2i²=-2; real 6-2=4, imaginary -4i-3i=-7i, so 4 - 7i. Denominator (3 + 2i)(3 - 2i)=9 +4=13. Thus (4 - 7i)/13 = 4/13 - 7/13 i. Choice A correctly rationalizes to 4/13 - 7/13 i. Choice B has +7/13 i, perhaps from sign error in imaginary part during multiplication—check FOIL signs carefully! Follow the division steps: identify conjugate, multiply top and bottom, expand, simplify. Example: (1 - i)/(1 + i) * (1 - i)/(1 - i) = (1 - 2i + i²)/(1 +1) = (1 - 2i -1)/2 = -2i/2 = -i—keep practicing, you're improving!

2

Multiply a complex number by its conjugate to get a real number: Compute $(4 + 3i)(4 - 3i)$. (Hint: $(a+bi)(a-bi)=a^2+b^2$.)

$7$

$25$

$16 - 9i^2$

$16 + 9i$

Explanation

This question tests your understanding of complex conjugates—pairs of complex numbers that differ only in the sign of their imaginary part—and how to use them to multiply to a real number. The conjugate of a + bi is a - bi; multiplying them gives (a + bi)(a - bi) = a² + b², a real positive result since imaginary parts cancel. For (4 + 3i)(4 - 3i), it's 4² + 3² = 16 + 9 = 25, using the formula directly. You could also expand: 44 = 16, 4(-3i) = -12i, 3i4 = 12i, 3i(-3i) = -9i² = 9 (since i² = -1), so 16 + 9 = 25. Choice C correctly applies the conjugate property to get the real number 25. Choice B forgets to simplify i² to -1, leaving it as 16 - 9i² without converting to 16 + 9 = 25—always remember i² = -1! The (a + bi)(a - bi) = a² + b² pattern is worth memorizing: it's the modulus squared and eliminates i. Practice with examples like (2 + i)(2 - i) = 4 + 1 = 5 to see the pattern—great job, keep practicing!

3

In an AC circuit, the impedance is $Z=4+3i$ ohms. A common calculation uses $Z\overline{Z}$, which equals the modulus squared. Compute $Z\overline{Z}$.

$25$

$7$

$1$

$16+9i$

Explanation

This question tests your understanding of complex conjugates—pairs of complex numbers that differ only in the sign of their imaginary part—and applying z $\overline{z}$ as the modulus squared in contexts like impedance. The conjugate of a + bi is a - bi; z $\overline{z}$ = a² + b², a real number representing |z|². For Z=4+3i, conjugate 4-3i; product 4² + 3²=16+9=25. In AC circuits, this relates to power calculations, but the math is the same. Choice C correctly computes 25. Choice D leaves it as 16+9i, perhaps forgetting to multiply or apply the formula—remember, the product is real! Use the quick formula a² + b² for any z $\overline{z}$. Try with 3+4i: 9+16=25, which is |z|²=5²— you're mastering this!

4

Let $z = -3 + 8i$. Which value equals $z\overline{z}$? (Recall $z\overline{z}=a^2+b^2$, the modulus squared.)

$55$

$-9 + 64$

$-73$

$73$

Explanation

This question tests your understanding of complex conjugates—pairs of complex numbers that differ only in the sign of their imaginary part—and that z $\overline{z}$ equals the modulus squared, a real number. The conjugate of a + bi is a - bi; z $\overline{z}$ = (a + bi)(a - bi) = a² + b². For z = -3 + 8i, conjugate is -3 - 8i; product (-3)² + 8² =9 +64=73. Expanding: (-3)(-3)=9, (-3)(-8i)=24i, 8i*(-3)=-24i, 8i*(-8i)=-64i²=64; 9+64=73. Choice C correctly gives 73, the real positive value. Choice A is -9 +64=55, but that's not the product; perhaps partial calculation without multiplying properly. Always use the a² + b² formula for z $\overline{z}$, noting the real part's sign doesn't matter since it's squared. Try with 1 + i: (1 + i)(1 - i)=1+1=2—perfect practice!

5

Let $z = 7 - 4i$. Which expression equals the complex conjugate $\overline{z}$ (recall: the conjugate flips the sign of the imaginary part only)?

$7 + 4i$

$7 - 4i$

$-7 - 4i$

$-7 + 4i$

Explanation

This question tests your understanding of complex conjugates—pairs of complex numbers that differ only in the sign of their imaginary part—and how to identify them correctly. The conjugate of a + bi is a - bi, flipping only the sign of the imaginary part while keeping the real part the same: for 7 - 4i, the conjugate is 7 + 4i. Remember, the magic property is that multiplying a complex number by its conjugate gives a real number, but here we're just identifying it. To find the conjugate of z = 7 - 4i, simply change the -4i to +4i, resulting in 7 + 4i. Choice B correctly identifies the conjugate by flipping only the imaginary part's sign. Choice A flips both signs, which is a common mistake—remember, only the imaginary part changes sign, not the real part! Conjugate quick reference: for a + bi, it's a - bi; for a - bi, it's a + bi—the real part stays the same. Practice by visualizing on the complex plane: the conjugate is the mirror image across the real axis, which helps build intuition for these problems—keep going, you're getting it!

6

Compute the product and note it is always real and nonnegative: $$(6-2i)(6+2i).$$

$40$

$36 - 4$

$32$

$36 + 4i$

Explanation

This question tests your understanding of complex conjugates—pairs of complex numbers that differ only in the sign of their imaginary part—and their product being real and nonnegative. The conjugate of a + bi is a - bi; their product is a² + b², always real and positive (or zero). For (6 - 2i)(6 + 2i), it's $6^2 + 2^2 = 36 + 4 = 40$. You can expand: $6 \times 6 = 36$, $6 \times 2i = 12i$, $-2i \times 6 = -12i$, $-2i \times 2i = -4i^2 = 4$; $36 + 4 = 40$, imaginaries cancel. Choice C correctly computes 40 using the formula. Choice A is 32, perhaps from 36 -4 instead of +4, forgetting i²=-1 makes -(-4)=+4. The pattern (a + bi)(a - bi) = a² + b² is key for modulus and rationalizing. Memorize it and practice with $(5 + i)(5 - i) = 25 + 1 = 26$—you've got this!

7

Compute $(4 + 3i)(4 - 3i)$. (Use the key property $(a+bi)(a-bi)=a^2+b^2$.)

$16 + 24i + 9i^2$

$25$

$7$

$16 - 9i^2$

Explanation

This question tests your understanding of complex conjugates—pairs of complex numbers that differ only in the sign of their imaginary part—and how multiplying by a conjugate yields a real number, specifically the modulus squared. The key property is (a + bi)(a - bi) = a² + b², where the imaginary parts cancel out, and since i² = -1, it simplifies to a real positive result: for instance, (3 + 2i)(3 - 2i) = 9 + 4 = 13. For (4 + 3i)(4 - 3i), identify a = 4 and b = 3, so it's 4² + 3² = 16 + 9 = 25, using the conjugate property directly without expanding fully. Choice C correctly applies this property to get 25, recognizing it's the square of the modulus |4 + 3i|². A tempting distractor like Choice B might stop at 16 - 9i² without simplifying i² to -1, but remember to always convert i² terms: 16 - 9(-1) = 16 + 9 = 25. Memorize the (a + bi)(a - bi) = a² + b² pattern—it's a shortcut for rationalizing denominators in division too! Keep practicing these multiplications, and you'll master complex numbers in no time.

8

In AC circuits, an impedance can be written as $Z=4+3i$ ohms. Compute $Z \overline{Z}$.

$16-9$

$16+9i$

$25$

$7i$

Explanation

This question tests your understanding of complex conjugates—pairs of complex numbers that differ only in the sign of their imaginary part—and applying the property $Z \overline{Z} = |Z|^2$, which is a real number representing the square of the modulus in contexts like AC circuits. For $Z = 4 + 3i$, the conjugate $\overline{Z} = 4 - 3i$, and $Z \overline{Z} = (4 + 3i)(4 - 3i) = 16 + 9 = 25$, using $(a + bi)(a - bi) = a^2 + b^2$. Choice C correctly computes this as 25, recognizing it's the modulus squared. A tempting distractor like Choice D might forget to simplify with $i^2 = -1$ and stop at 16 - 9, but always remember the conjugate product eliminates imaginaries to give a real result. Use this pattern in physics or engineering: $|Z| = \sqrt{a^2 + b^2}$, so $|Z|^2 = a^2 + b^2$ directly. You're making fantastic progress—try it with $Z = 5 + 12i$ to get $25 + 144 = 169$!

9

Let $z = 7 - 4i$. What is the complex conjugate $\overline{z}$? (Recall: the conjugate of $a+bi$ is $a-bi$.)

$7 - 4i$

$-7 + 4i$

$-7 - 4i$

$7 + 4i$

Explanation

This question tests your understanding of complex conjugates—pairs of complex numbers that differ only in the sign of their imaginary part—and how to identify them directly from the definition. The conjugate of a + bi is a - bi, where you flip only the sign of the imaginary part while keeping the real part the same: for example, the conjugate of 3 + 4i is 3 - 4i, and for 2 - 5i, it's 2 + 5i. For z = 7 - 4i, the conjugate is 7 + 4i because we change -4i to +4i, leaving the real part 7 unchanged—this is like reflecting the point across the real axis in the complex plane. Choice B correctly identifies the conjugate by flipping only the imaginary part's sign, resulting in 7 + 4i. A tempting distractor like Choice A or C might flip the signs of both parts, but remember, only the imaginary part's sign changes—the real part stays the same, so -7 + 4i or -7 - 4i would be incorrect. To find the conjugate quickly, always keep the real number as is and switch the sign before the i; practicing with a few examples like 1 + i (conjugate 1 - i) or -3 - 2i (conjugate -3 + 2i) will make this second nature. You're doing great—keep visualizing that mirror flip across the real axis!

10

Let $z=2+5i$ and $w=1-3i$. Find $\overline{(z+w)}$.

$3-2i$

$1-8i$

$3+2i$

$1+8i$

Explanation

This question tests your understanding of complex conjugates—pairs of complex numbers that differ only in the sign of their imaginary part—and the property that the conjugate of a sum is the sum of the conjugates: $\overline{z + w}$ = $\overline{z}$ + $\overline{w}$. The conjugate of a + bi is a - bi; for z = 2 + 5i, $\overline{z}$ = 2 - 5i; for w = 1 - 3i, $\overline{w}$ = 1 + 3i. To find $\overline{(z + w)}$, first add z + w = (2 + 1) + (5 - 3)i = 3 + 2i, then take conjugate: 3 - 2i. Choice A correctly computes this by adding first and then conjugating, or equivalently adding the conjugates: (2 - 5i) + (1 + 3i) = 3 - 2i. A tempting distractor like Choice B might forget to flip the sign for the conjugate of the sum, giving 3 + 2i instead, but remember, conjugation always flips the imaginary sign. The transferable strategy is to apply properties like $\overline{z + w}$ = $\overline{z}$ + $\overline{w}$ to simplify calculations—try it with subtraction too: $\overline{z - w}$ = $\overline{z}$ - $\overline{w}$. Great job exploring these properties; they make working with complexes much easier!

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