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Algebra

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Find constants A and B such that

$\frac{x-7}{x^2 -x-2}=\frac{A}{x-2} + \frac{B}{x+1}$

for all x such that $x \neq -1$ and $x \neq 2$. Give your answer as the ordered pair (A,B).

Jul 12, 2021

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Answer: $$(-\frac5 3,\frac8 3)$$

Solution:

The right side of the equation can be turned into $$A(x+1) + B(x-2)\over x^2 -x-2$$. After that, you can multiply both sides of the equation to get $$x-7 = A(x+1) + B(x-2)$$.

The coefficient of the x on the left, which is one, is equal to A + B, because the x term on the left side is made up of Ax + Bx. The constant term on the left side, -7, is equal to A-2B. This is because when expanding out the terms on the right, you would get Ax +$$A$$+Bx $$-2B$$. From this information, you get two equations:

$$A+B=1$$

and

$$A-2B=-7$$

Subtracting the second equation from the first gives 3B = 8, which means that B = 8/3. Substituting this value into the first equation gives A + 8/3 = 1. Solving this gives A = 1-8/3 = -5/3.

Putting this into an ordered pair gives $$(-\frac5 3,\frac8 3)$$.

Jul 12, 2021