Polynomial Functions and Complex Zeros

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AP Precalculus › Polynomial Functions and Complex Zeros

Questions 1 - 10
1

Based on the passage, the degree-2 polynomial $v(x)=8x^2+16x+13$ has $\Delta=256-416=-160$; calculate the complex zeros using the quadratic formula and simplify $\sqrt{-160}=4\sqrt{10},i$.​

$x=-1\pm\dfrac{\sqrt{10}}{4}i$

$x=-1+\dfrac{\sqrt{10}}{4}i$ only

$x=-1\pm\dfrac{\sqrt{10}}{2}i$

$x=1\pm\dfrac{\sqrt{10}}{4}i$

$x=-2\pm\dfrac{\sqrt{10}}{4}i$

Explanation

This question tests AP Precalculus skills: understanding polynomial functions and complex zeros. Complex zeros occur in conjugate pairs when a quadratic has a negative discriminant. For v(x) = 8x² + 16x + 13 with Δ = -160, we use: x = (-16 ± √(-160))/(2·8) = (-16 ± √160 i)/16. Since √160 = √(16·10) = 4√10, we have √(-160) = 4√10 i, giving x = (-16 ± 4√10 i)/16 = -1 ± (√10/4)i. Choice A is correct because it shows the properly simplified form with real part -1 and imaginary parts ±√10/4. Choice B is incorrect because it shows ±√10/2, which would result from dividing by 8 instead of 16. To help students: Always simplify radicals by factoring out perfect squares, then reduce fractions carefully. Watch for: Confusion about whether to divide by 2a or just a in the quadratic formula.

2

Using the polynomial provided, a quadratic $u(x)=3x^2-18x+31$ has $\Delta=324-372=-48$; calculate the complex zeros with the quadratic formula, rewriting $\sqrt{-48}=4\sqrt{3},i$ and simplifying.​

$x=3+\dfrac{2\sqrt{3}}{3}i$ only

$x=\dfrac{18\pm4\sqrt{3}}{6}$

$x=3\pm\dfrac{4\sqrt{3}}{3}i$

$x=-3\pm\dfrac{2\sqrt{3}}{3}i$

$x=3\pm\dfrac{2\sqrt{3}}{3}i$

Explanation

This question tests AP Precalculus skills: understanding polynomial functions and complex zeros. For quadratics with negative discriminants, the zeros are complex conjugates found via the quadratic formula. Given u(x) = 3x² - 18x + 31 with Δ = -48, we calculate: x = (18 ± √(-48))/(2·3) = (18 ± √48 i)/6. Since √48 = √(16·3) = 4√3, we have √(-48) = 4√3 i, giving x = (18 ± 4√3 i)/6 = 3 ± (2√3/3)i. Choice A is correct because it properly simplifies to the standard form a ± bi with rationalized denominators. Choice B is incorrect because it shows ±4√3/3, failing to simplify the fraction 4/6 to 2/3. To help students: Factor perfect squares from radicals before reducing fractions, and always simplify completely. Watch for: Missing the simplification of 4/6 to 2/3 in the imaginary part.

3

Based on the passage, let $f(x)=x^3+2x^2+5x+10$, a degree $3$ polynomial. Factor by grouping: $x^3+2x^2+5x+10=x^2(x+2)+5(x+2)=(x+2)(x^2+5)$. The real zero is $x=-2$, and the remaining zeros satisfy $x^2=-5$, so $x=\pm\sqrt{5},i$. In the complex plane, $\pm\sqrt{5}i$ lie on the imaginary axis, symmetric about the real axis. What are the complex zeros of the polynomial function $f(x)=x^3+2x^2+5x+10$?

$x=-2,\ \pm\sqrt{5},i$

$x=-2,\ \pm\sqrt{5}$

$x=2,\ \pm\sqrt{5},i$

$x=-2,\ \sqrt{5},i$

$x=-2,\ \pm 5i$

Explanation

This question tests AP Precalculus skills: understanding polynomial functions and complex zeros. Factoring by grouping is an efficient method for certain polynomials, revealing both real and complex zeros. For f(x) = x³ + 2x² + 5x + 10, grouping gives x²(x+2) + 5(x+2) = (x+2)(x²+5). The real zero is x = -2, and from x² + 5 = 0, we get x² = -5, so x = ±√(5)i. Choice A is correct because it identifies all three zeros: the real zero -2 and the conjugate pair ±√(5)i. Choice B is incorrect because it has the wrong sign for the real zero (2 instead of -2). To help students: Practice recognizing grouping patterns and connecting factored form to zeros. Emphasize that x² + positive number = 0 always yields purely imaginary zeros.

4

Based on the passage, the degree-2 polynomial $p(x)=4x^2+4x+5$ has $\Delta=16-80=-64$; calculate the complex zeros using the quadratic formula and simplify $\sqrt{-64}=8i$ to obtain exact $a+bi$ values.​

$x=-\dfrac{1}{2}\pm i$

$x=-1\pm i$

$x=\dfrac{1}{2}\pm i$

$x=-\dfrac{1}{2}\pm2i$

$x=-\dfrac{1}{2}+i$ only

Explanation

This question tests AP Precalculus skills: understanding polynomial functions and complex zeros. Complex zeros of polynomials with real coefficients always come in conjugate pairs a ± bi. For p(x) = 4x² + 4x + 5 with Δ = -64, we apply the quadratic formula: x = (-4 ± √(-64))/(2·4) = (-4 ± 8i)/8. Simplifying by dividing both terms by 8, we get x = -1/2 ± i. Choice A is correct because it shows the properly reduced form with real part -1/2 and imaginary parts ±i. Choice B is incorrect because it shows -1/2 ± 2i, which would come from incorrectly simplifying (-4 ± 16i)/8. To help students: Remember that √(-64) = 8i (not 16i), and always reduce fractions completely. Watch for: Confusion between √64 = 8 and mistakenly doubling it when dealing with imaginary numbers.

5

Using the polynomial provided, a quadratic model $h(x)=x^2-6x+11$ has $\Delta=36-44=-8$; calculate the complex zeros using the quadratic formula, rewriting $\sqrt{-8}=2\sqrt{2},i$ and expressing answers as $a\pm bi$.​

$x=3+\sqrt{2},i$ only

$x=-3\pm\sqrt{2},i$

$x=3\pm\sqrt{2},i$

$x=3\pm2\sqrt{2},i$

$x=\dfrac{6\pm\sqrt{2}}{2}$

Explanation

This question tests AP Precalculus skills: understanding polynomial functions and complex zeros. When a quadratic has a negative discriminant, it yields two complex conjugate zeros that can be found using the quadratic formula. For h(x) = x² - 6x + 11 with Δ = -8, we calculate: x = (6 ± √(-8))/(2·1) = (6 ± √8 i)/2. Since √8 = √(4·2) = 2√2, we have √(-8) = 2√2 i, giving x = (6 ± 2√2 i)/2 = 3 ± √2 i. Choice A is correct because it shows the simplified form with real part 3 and imaginary parts ±√2 i. Choice B is incorrect because it shows 3 ± 2√2 i, failing to divide the coefficient of i by 2. To help students: Practice simplifying square roots by factoring out perfect squares, and always divide both the real and imaginary parts by 2a. Watch for: Errors in simplifying √8 and forgetting to divide the entire expression by 2.

6

Using the polynomial provided, consider $f(x)=x^2+4x+13$, a degree $2$ polynomial. Completing the square gives $x^2+4x+13=(x+2)^2+9$, so $f(x)=0$ implies $(x+2)^2=-9$. Taking square roots, $x+2=\pm 3i$, hence $x=-2\pm 3i$. These nonreal zeros do not create $x$-intercepts on the real coordinate plane, yet they still satisfy the equation in the complex number system. On the complex plane, they plot at $(-2,\pm 3)$ using real and imaginary axes. What are the complex zeros of the polynomial function $f(x)=x^2+4x+13$?

$x=-2\pm 9i$

$x=-2+3i$

$x=-2\pm 3$

$x=-2\pm 3i$

$x=2\pm 3i$

Explanation

This question tests AP Precalculus skills: understanding polynomial functions and complex zeros. Completing the square is an alternative method to find complex zeros when the discriminant is negative. For f(x) = x² + 4x + 13, completing the square gives (x+2)² + 9 = 0, so (x+2)² = -9. Taking square roots yields x + 2 = ±3i, therefore x = -2 ± 3i. Choice B is correct because it shows the proper form with real part -2 and imaginary parts ±3i. Choice A is incorrect because it has the wrong real part (2 instead of -2). To help students: Practice completing the square and connecting it to the quadratic formula. Visualize complex zeros on the complex plane to reinforce that they don't create x-intercepts on the real coordinate system.

7

Based on the passage, the degree-2 polynomial $t(x)=6x^2-12x+11$ has $\Delta=144-264=-120$; calculate the complex zeros using the quadratic formula and simplify $\sqrt{-120}=2\sqrt{30},i$.​

$x=1\pm\dfrac{\sqrt{30}}{6}i$

$x=1+\dfrac{\sqrt{30}}{6}i$ only

$x=1\pm\dfrac{\sqrt{30}}{3}i$

$x=-1\pm\dfrac{\sqrt{30}}{6}i$

$x=\dfrac{12\pm2\sqrt{30}}{6}$

Explanation

This question tests AP Precalculus skills: understanding polynomial functions and complex zeros. Complex zeros of quadratics with real coefficients come in conjugate pairs when the discriminant is negative. For t(x) = 6x² - 12x + 11 with Δ = -120, we apply: x = (12 ± √(-120))/(2·6) = (12 ± √120 i)/12. Since √120 = √(4·30) = 2√30, we have √(-120) = 2√30 i, giving x = (12 ± 2√30 i)/12 = 1 ± (√30/6)i. Choice A is correct because it shows the fully reduced form with proper simplification of both parts. Choice B is incorrect because it shows ±√30/3, which would come from dividing by 6 instead of 12. To help students: Always simplify radicals first, then reduce the entire fraction. Watch for: Errors in simplifying √120 and confusion about which denominator to use.

8

Based on the passage, let $f(x)=x^4-2x^3+5x^2-8x+4$, a degree $4$ polynomial with four zeros counting multiplicity. Suppose $x=1$ is a zero. Synthetic division by $(x-1)$ gives coefficients $1,-2,5,-8,4$ and produces the quotient $x^3-x^2+4x-4$ with remainder $0$. Factor the cubic by grouping: $x^3-x^2+4x-4=x^2(x-1)+4(x-1)=(x-1)(x^2+4)$. Therefore $f(x)=(x-1)^2(x^2+4)$. Solving $x^2+4=0$ yields $x=\pm 2i$, which appear as conjugates. What are the complex zeros of the polynomial function $f(x)=x^4-2x^3+5x^2-8x+4$?

$x=1,1,\ \pm 4i$

$x=1,1,\ \pm 2i$

$x=-1,-1,\ \pm 2i$

$x=1,\ \pm 2i$

$x=1,1,\ 2i$

Explanation

This question tests AP Precalculus skills: understanding polynomial functions and complex zeros. A degree 4 polynomial has exactly four zeros counting multiplicity, and the problem demonstrates synthetic division and factoring techniques. Starting with f(x) = x⁴ - 2x³ + 5x² - 8x + 4 and knowing x = 1 is a zero, synthetic division yields x³ - x² + 4x - 4. Factoring by grouping gives (x-1)(x²+4), so f(x) = (x-1)²(x²+4). Choice A is correct because it lists all four zeros with proper multiplicity: x = 1 with multiplicity 2, and x = ±2i. Choice B is incorrect because it only lists three zeros, missing the multiplicity of x = 1. To help students: Practice synthetic division and recognize repeated factors. Emphasize counting zeros with their multiplicities to match the polynomial's degree.

9

Based on the passage, consider the quartic $f(x)=x^4+2x^3+5x^2+10x+13$, degree $4$. Suppose the polynomial factors as $(x^2+ax+b)(x^2+cx+d)$ with real coefficients, so nonreal zeros occur in conjugate pairs. Matching constant terms requires $bd=13$, and matching the $x^3$ coefficient requires $a+c=2$. The passage provides the completed-square style factorization $f(x)=(x^2+2x+5)(x^2+1)$. Solve $x^2+1=0$ to get $x=\pm i$, and solve $x^2+2x+5=0$ to get $x=\dfrac{-2\pm\sqrt{4-20}}{2}=-1\pm 2i$. What are the complex zeros of the polynomial function $f(x)=x^4+2x^3+5x^2+10x+13$?

$x=\pm i,\ -1\pm 2i$

$x=\pm i,\ 1\pm 2i$

$x=\pm 1,\ -1\pm 2i$

$x=\pm 2i,\ -1\pm i$

$x=i,\ -1\pm 2i$

Explanation

This question tests AP Precalculus skills: understanding polynomial functions and complex zeros. A quartic polynomial has four zeros, and when factored into quadratics with real coefficients, each quadratic may yield a conjugate pair of complex zeros. The polynomial f(x) = x⁴ + 2x³ + 5x² + 10x + 13 factors as (x² + 2x + 5)(x² + 1). From x² + 1 = 0, we get x = ±i; from x² + 2x + 5 = 0, using the quadratic formula gives x = (-2 ± √(-16))/2 = -1 ± 2i. Choice A is correct because it lists all four zeros: ±i and -1 ± 2i. Choice D is incorrect because it has the wrong real part for the second pair (1 instead of -1). To help students: Practice factoring quartics into quadratics and solving each factor separately. Verify factorizations by expanding to check coefficients.

10

Based on the passage, a degree-2 polynomial $g(x)=3x^2+12x+17$ has $\Delta=144-204=-60$; calculate the complex zeros of $g(x)$ using the quadratic formula and simplify $\sqrt{-60}=2\sqrt{15},i$.​

$x=-2\pm\sqrt{15},i$

$x=2\pm\dfrac{\sqrt{15}}{3}i$

$x=-2\pm\dfrac{\sqrt{15}}{3}i$

$x=-2+\dfrac{\sqrt{15}}{3}i$ only

$x=-2\pm\dfrac{\sqrt{15}}{6}i$

Explanation

This question tests AP Precalculus skills: understanding polynomial functions and complex zeros. For a quadratic with negative discriminant, the zeros are complex conjugates a ± bi where both a and b are real numbers. Given g(x) = 3x² + 12x + 17 with Δ = -60, we use the quadratic formula: x = (-12 ± √(-60))/(2·3) = (-12 ± √60 i)/6. Since √60 = √(4·15) = 2√15, we have √(-60) = 2√15 i, giving x = (-12 ± 2√15 i)/6 = -2 ± (√15/3)i. Choice A is correct because it accurately shows both the real part (-2) and the properly simplified imaginary part (±√15/3). Choice D is incorrect because it shows -2 ± √15 i, failing to divide the imaginary coefficient by 3. To help students: Always simplify radicals by factoring out perfect squares, and ensure both parts of the complex number are divided by 2a. Watch for: Common errors in simplifying the imaginary part and forgetting to reduce fractions.

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