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The roots of $7x^2 + x - 5 = 0$ are $a$ and $b.$ Compute $(a - 4)(b - 4).$[tex]The roots of $7x^2 + x - 5 = 0$ are $a$ and $b.$ Compute $(a - 4)(b - 4).$[/tex]

Respuesta :

LammettHash

Using the factor theorem, we have

[tex]7x^2+x-5=7(x-a)(x-b)[/tex]

and expanding gives us

[tex]7x^2+x-5=7(x^2-(a+b)x+ab)\implies\begin{cases}ab=-5\\a+b=-1\end{cases}[/tex]

So we have

[tex](a-4)(b-4)=ab-4(a+b)+16=-5-4(-1)+16=\boxed{15}[/tex]

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