Precalculus : Find Complex Zeros of a Polynomial Using the Fundamental Theorem of Algebra

Study concepts, example questions & explanations for Precalculus

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Example Questions

Example Question #431 : Pre Calculus

Find a fourth-degree polynomial whose zeroes are \(\displaystyle -2,5\), and \(\displaystyle -2\pm \sqrt{2i}\)

Possible Answers:

\(\displaystyle x^{4}-x^{3}-16x^{2}-58x-60\)

\(\displaystyle x^{4}+x^{3}-16x^{2}-58x-60\)

Correct answer:

\(\displaystyle x^{4}+x^{3}-16x^{2}-58x-60\)

Explanation:

This one is a bit of a journey. The expressions for the first two zeroes are easily calculated, \(\displaystyle \left ( x+2 \right )\) and \(\displaystyle \left ( x-5 \right )\) respectively. The last expression must be broken up into two equations: \(\displaystyle x=-2-i\sqrt{2},x=-2+i\sqrt{2}\) which are then set equal to zero to yield the expressions \(\displaystyle \left ( x+2-i\sqrt{2} \right )\) and \(\displaystyle \left ( x+2+i\sqrt{2} \right )\)

Finally, we multiply together all of the parenthesized expressions, which multiplies out to \(\displaystyle x^{4}+x^{3}-16x^{2}-58x-60=0\)

Example Question #432 : Pre Calculus

The third-degree polynomial expression \(\displaystyle f(x)=x^{3}-x^{2}+4x-4\) has a real zero at \(\displaystyle x=1\). Find all of the complex zeroes.

Possible Answers:
Correct answer:

\(\displaystyle x=\pm 2i\)

Explanation:

First, factor the expression by grouping: \(\displaystyle x^{3}-x^{2}+4x-4\rightarrow x^{2}(x-1)+4(x-1)\rightarrow (x^{2}+4)\cdot (x-1)\)

To find the complex zeroes, set the term \(\displaystyle (x^{2}+4)\) equal to zero: \(\displaystyle x^{2}=-4\rightarrow x=\pm \sqrt{-2}=\pm 2i\)

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