Partial Fractions Are Fun!

Algebra Level 3

If x 2 + 2 ( x 1 ) ( x 2 ) ( x 3 ) \dfrac{x^{2}+2}{(x-1)(x-2)(x-3)} can be written as A ( x 1 ) + B ( x 2 ) + C ( x 3 ) \dfrac{A}{(x-1)}+\dfrac{B}{(x-2)}+\dfrac{C}{(x-3)} then,

What is the value of B B ?


The answer is -6.

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1 solution

Zach Abueg
Jul 13, 2017

x 2 + 2 ( x 1 ) ( x 2 ) ( x 3 ) = A x 1 + B x 2 + C x 3 Multiply throughout by ( x 1 ) ( x 2 ) ( x 3 ) A ( x 2 ) ( x 3 ) + B ( x 1 ) ( x 3 ) + C ( x 1 ) ( x 2 ) = x 2 + 2 Let x = 2 B ( 2 1 ) ( 2 3 ) = 2 2 + 2 B = 6 \displaystyle \begin{aligned} \frac{x^2 + 2}{(x - 1)(x - 2)(x - 3)} & = \frac{A}{x - 1} + \frac{B}{x - 2} + \frac{C}{x - 3} & \small \color{#3D99F6} \text{Multiply throughout by } (x - 1)(x - 2)(x - 3) \\ \implies A(x - 2)(x - 3) + B(x - 1)(x - 3) + C(x - 1)(x - 2) & = x^2 + 2 & \small \color{#3D99F6} \text{Let } x = 2 \\ \implies B(2 - 1)(2 - 3) & = 2^2 + 2 \\ \implies B & = \boxed{- 6} \end{aligned}

After finding A A and C C in a similar fashion, our decomposed fraction is

3 2 ( x 1 ) 6 x 2 + 11 2 ( x 3 ) \displaystyle \begin{aligned} \frac{3}{2(x - 1)} - \frac{6}{x - 2} + \frac{11}{2(x - 3)} \end{aligned}

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