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Evaluate ∫2x+1(x+1)(x-2) dx - Mathematics and Statistics

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Question

Evaluate `int (2x + 1)/((x + 1)(x - 2))  "d"x`

Sum

Solution

Let I = `int (2x + 1)/((x + 1)(x - 2))  "d"x`

Let `(2x + 1)/((x + 1)(x - 2)) = "A"/(x + 1) + "B"/(x - 2)`

∴ 2x + 1 = A(x – 2) + B(x + 1)   ......(i)

Putting x = – 1 in (i), we get

2(– 1) + 1 = A(– 1 – 2) + B(0)

∴ – 1 = – 3A

∴ A = `1/3`

Putting x = 2 in (i), we get

2(2) + 1 = A(0) + B(2 + 1)

∴ 5 = 3B

∴ B = `5/3`

∴ `(2x + 1)/((x + 1)(x - 2)) = ((1/3))/(x + 1) + ((5/3))/(x - 2)`

∴ I = `int(((1/3))/(x + 1) + ((5/3))/(x - 2))  "d"x`

= `1/3 int 1/(x + 1)  "d"x + 5/3 int 1/(x - 2)  "d"x`

∴ I = `1/3 log|x + 1| + 5/3 log|x - 2| + "c"`

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Chapter 1.5: Integration - Q.4
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