Partial Fractions

Algebra Level 3

1 a x 2 + b x + c = A x b + b 2 4 a c 2 a + B x b b 2 4 a c 2 a \dfrac{1}{ax^2 + bx + c } = \dfrac{A}{x - \dfrac{-b + \sqrt{b^2 - 4ac}}{2a}} + \dfrac{B}{x - \dfrac{-b - \sqrt{b^2 - 4ac}}{2a}}

Assume that a x 2 + b x + c 0 ax^2 + bx + c \neq 0 . Find A + B A+B .


The answer is 0.

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1 solution

Hobart Pao
Jun 11, 2015

The roots of a x 2 + b x + c ax^2 +bx+c can be represented as b + b 2 4 a c 2 a \dfrac{-b+\sqrt{b^2 -4ac}}{2a} and b b 2 4 a c 2 a \dfrac{-b-\sqrt{b^2 -4ac}}{2a} . Thus, the partial fractions will be written like A x b + b 2 4 a c 2 a + B x b b 2 4 a c 2 a \displaystyle\frac{A}{x- \dfrac{-b+\sqrt{b^2 -4ac}}{2a}} + \frac{B}{x- \dfrac{-b-\sqrt{b^2 -4ac}}{2a}} . Rewrite as B ( x b + b 2 4 a c 2 a ) + A ( x b b 2 4 a c 2 a ) = 1 B \left( x- \dfrac{-b+\sqrt{b^2 -4ac}}{2a} \right) + A \left( x- \dfrac{-b-\sqrt{b^2 -4ac}}{2a}\right) =1 . Substitute b + b 2 4 a c 2 a \dfrac{-b+\sqrt{b^2 -4ac}}{2a} you will get A ( b 2 4 a c a ) = 1 A\left(\dfrac{\sqrt{b^2 -4ac}}{a}\right)=1 , which gives A as a b 2 4 a c \dfrac{a}{\sqrt{b^2 - 4ac}} . Do the same except substitute b b 2 4 a c 2 a \dfrac{-b-\sqrt{b^2 -4ac}}{2a} and you will get B as a b 2 4 a c -\dfrac{a}{\sqrt{b^2 - 4ac}} . The sum of A and B are zero, and you will see that I put a=2014, b=2015, and c=2016 for show.

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