Math Help Please
posted by Jacinta on .
Suppose that a polynomial function of degree 5 with rational coefficients has 0 (with multiplicity 2), 3, and 1 –2i as zeros. Find the remaining zero.
A. –2
B. –1 – 2i
C. 0
D. 1 + 2i

So the funtion is:
x^2(x  7)(x + 3  5i)(x  3 + 5i)
For it to have rational coefficients, you must get rid of the 3 + 5i term.
Therefore the remaining zero is:
3  5i
so your answer would be (C) 
How can the answer be 0 ? I don't get it, I figured to be 12i Choice B. What did i do wrong? Thanks!

There is a little rule of thumb that you can count on for finding zeroes of polynomials with complex roots.
Complex roots always come in pairs. Each root of the pair is the complex conjugate of the other.
For example, if you have a root as 4+3i, the other root must be 43i. If another complex root is 2i, then its conjugate is 2+i, etc.
To get the correct answer to the given problem, you only need to choose the complex conjugate of the given complex root. 
The function would be
f(x)=x²(x3)(x1+2i)(x12i)
= x^{5}5x^{4}+11x³15x² 
7+what equal 125