An algebra problem by A Former Brilliant Member

Algebra Level 2

Which option corresponds to the partial fraction decomposition of the rational function 13 6 x 2 + 5 x + 6 \frac{13}{-6x^2 + 5x + 6} ?

2 2 x 3 3 3 x + 2 -\frac{2}{2x - 3} - \frac{3}{3x + 2} 2 2 x 3 + 3 3 x + 2 \frac{2}{2x - 3} +\frac{3}{3x + 2} 2 2 x 3 + 3 3 x + 2 -\frac{2}{2x - 3} +\frac{3}{3x + 2} 2 2 x 3 3 3 x + 2 \frac{2}{2x - 3} -\frac{3}{3x + 2}

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1 solution

To decompose the fraction, we factor the denominator and obtain,

13 ( 2 x + 3 ) ( 3 x + 2 ) \frac{13}{(-2x + 3)(3x + 2)}

Therefore we have 13 ( 2 x + 3 ) ( 3 x + 2 ) = \frac{13}{(-2x + 3)(3x + 2)} = A 2 x + 3 + \frac{A}{-2x + 3} + B 3 x + 2 \frac{B}{3x + 2}

The equation above is an identity for all x x except 3 2 \frac{3}{2} and 2 3 \frac{-2}{3} . By multiplying both sides on the LCD we obtain,

13 = A ( 3 x + 2 ) + B ( 2 x + 3 ) 13 = A(3x + 2) + B(-2x + 3)

We now find the constants A A and B B , substituting 3 2 \frac{3}{2} to the above equation, we get A = 2 A=2

Substituting 2 3 \frac{-2}{3} , we get B = 3 B=3

With these values for A A and B B ,

13 ( 2 x + 3 ) ( 3 x + 2 ) = \frac{13}{(-2x + 3)(3x + 2)} = - 2 2 x 3 + \frac{2}{2x - 3} + 3 3 x + 2 \frac{3}{3x + 2}

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