There are integers a and b such that the polynomial x3β5x2+ax+b has 4+5β as a root. What is a+b?
Answer Choices:
A. 13
B. 17
C. 20
D. 30
E. 68
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Since 4+5β is a root and all the coefficients are integers, the conjugate 4β5β must also be a root. By Vieta's, the roots sum to 5, so the last root must be β3. Hence the polynomial must be:
(x2β8x+11)(x+3)=x3β5x2β13x+33
and so (a,b)=(β13,33). This yields a final answer of (C)20β.
Alternate Solution:
Suppose P(x) was the desired polynomial. Then P(1)=a+bβ4. We found the roots to be 4+5β, 4β5β, and β3, so we could calculate: