Suppose $$\frac{2(1-2x)}{x^2-x-2} = \frac{A}{x-2} + \frac{B}{x+1}$$

How can I get the value of $A+B$ for my math exam?

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For this question to have an answer, it should be supposed that $x$ is an unknown. Notice that $x^2 - x -2 = (x-2)(x +1)$. So, what will you get if you multiply both sides by $(x-2)(x+1)$? Try to get an equation of the form $(\cdot)x + (\cdot) = 0$. Then the things in the $()$'s must both be $0$ for the equation to hold. You know how to do it now? –  ShinyaSakai Nov 13 '11 at 16:56
If it helps, here you can find a few solved exercises of this type: tutorial.math.lamar.edu/Classes/CalcII/PartialFractions.aspx –  Martin Sleziak Nov 13 '11 at 17:36

This should help. $$2 - 4x = Ax + A + Bx - 2B = (A+B) x + (A-2B)$$