2006 AMC 10B Problems/Problem 14

Problem

Let $a$ and $b$ be the roots of the equation $x^2-mx+2=0$ . Suppose that $a+(1/b)$ and $b+(1/a)$ are the roots of the equation $x^2-px+q=0$ . What is $q$ ?

$\mathrm{(A) \ } \frac{5}{2}\qquad \mathrm{(B) \ } \frac{7}{2}\qquad \mathrm{(C) \ } 4\qquad \mathrm{(D) \ } \frac{9}{2}\qquad \mathrm{(E) \ } 8$

Solution

In a quadratic equation in the form $x^2 + bx + c = 0$ , the product of the roots is $c$

Using this property:

$ab=2$

$q = (a+\frac{1}{b})\cdot(b+\frac{1}{a}) = (\frac{ab+1}{b})\cdot(\frac{ab+1}{a}) = \frac{(ab+1)^2}{ab} = \frac{(2+1)^2}{2} = \frac{9}{2} \Rightarrow D$

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2006 AMC 10B Problems/Problem 14

Problem

Solution

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