partial FRACTION!!!!!

Algebra Level 2

The partial fraction 6 x + 1 4 x 2 + 4 x 3 \frac{6x+1}{4x^{2} +4x -3} can be written as:

α 2 x + 3 \frac{ \alpha }{2x+3} + + β 2 x 1 \frac{ \beta }{2x-1}

what is the value of α + β \alpha + \beta ?


The answer is 3.

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1 solution

Scott Cambo
Feb 8, 2017

Long time problem solver, first time solution provider. Let me know if I can clarify.

Step 1 : Typically partial fraction decomposition begins with combining the two fractions into one.

α 2 x + 3 + β 2 x 1 \frac{\alpha}{2x+3} + \frac{\beta}{2x-1} = α ( 2 x 1 ) ( 2 x + 3 ) ( 2 x 1 ) + β ( 2 x + 3 ) ( 2 x + 3 ) ( 2 x 1 ) \frac{\alpha(2x-1)}{(2x+3)(2x-1)} + \frac{\beta(2x+3)}{(2x+3)(2x-1)}

= α ( 2 x 1 ) + β ( 2 x + 3 ) ( 2 x + 3 ) ( 2 x 1 ) = \frac{\alpha(2x-1) + \beta(2x+3)}{(2x+3)(2x-1)}

Step 2 : Now, to the extent possible, we want to make this new fraction look as similar to the one we started with is as possible. This will help us to determine what α \alpha and β \beta should be.

α ( 2 x 1 ) + β ( 2 x + 3 ) ( 2 x + 3 ) ( 2 x 1 ) = α 2 x α + β 2 x + 3 β ( 2 x + 3 ) ( 2 x 1 ) \frac{\alpha(2x-1) + \beta(2x+3)}{(2x+3)(2x-1)} = \frac{\alpha \cdot 2x - \alpha + \beta \cdot 2x + 3\beta}{(2x+3)(2x-1)} )

= α 2 x + β 2 x α + 3 β ( 2 x + 3 ) ( 2 x 1 ) = \frac{\alpha \cdot 2x + \beta \cdot 2x - \alpha + 3\beta}{(2x+3)(2x-1)} )

= 2 x ( α + β ) α + 3 β ( 2 x + 3 ) ( 2 x 1 ) = \frac{2x(\alpha + \beta) - \alpha + 3\beta}{(2x+3)(2x-1)} )

We now have something pretty close where the x x has α \alpha and β \beta constants.

Step 3 : We can now start to make some comparisons.

Let's think of 2 x ( α + β ) 2x(\alpha + \beta) as equal to 6x) and see if we can solve for just \(\alpha and β \beta

2 x ( α + β ) = 6 x 2x(\alpha + \beta) = 6x

divide both sides by 2 x 2x and we have : α + β = 3 \alpha + \beta = 3

That's what we want! 3. We could keep going to solve for α \alpha and β \beta , but we have our answer.

Why are we equating 2x( a+b ) to 6x? Where did 6x come from?

Gigi Vega - 2 years, 7 months ago

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