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Integrals

Question
CBSEENMA12032432

Prove the following:
integral subscript 1 superscript 3 fraction numerator dx over denominator straight x squared left parenthesis straight x plus 1 right parenthesis end fraction space equals space 2 over 3 plus log 2 over 3


Solution

Let I = integral subscript 1 superscript 3 fraction numerator 1 over denominator straight x squared left parenthesis straight x plus 1 right parenthesis end fraction dx
Put fraction numerator 1 over denominator straight x squared left parenthesis straight x plus 1 right parenthesis end fraction space equals space straight A over straight x plus straight B over straight x squared plus fraction numerator straight C over denominator straight x plus 1 end fraction
Multiplying both sides by x2 (x + 1), we get,
1 ≡ A x (x + 1) + B (x + 1) + C x2    ...(1)
Putting x = 0 in (1), we get,
1 = B, ∴ B = 1
Putting x + 1 = 0 or x = – 1 in (1), we get,
1 = C (– 1 )2, ∴ C = 1
(1) can be written as
1 ≡ A (x+ x) + B(x + 1) + C x2    ...(2)
Equating coeffs. in (2) of x2, we get,
A + C = 0 ⇒ A + 1 = 0 ⇒ A = – 1
therefore     fraction numerator 1 over denominator straight x squared left parenthesis straight x plus 1 right parenthesis end fraction space equals space 1 over straight x plus 1 over straight x squared plus fraction numerator 1 over denominator straight x plus 1 end fraction
therefore   I = integral subscript 1 superscript 3 open parentheses negative 1 over straight x plus 1 over straight x squared plus fraction numerator 1 over denominator straight x plus 1 end fraction close parentheses dx space equals space integral subscript 1 superscript 3 open parentheses negative 1 over straight x plus straight x to the power of negative 2 end exponent plus fraction numerator 1 over denominator straight x plus 1 end fraction close parentheses dx
space equals open square brackets negative logx plus fraction numerator straight x to the power of negative 1 end exponent over denominator negative 1 end fraction plus log left parenthesis straight x plus 1 right parenthesis close square brackets subscript 1 superscript 3 space equals space open square brackets negative logx minus 1 over straight x plus log left parenthesis straight x plus 1 right parenthesis close square brackets subscript 1 superscript 3
equals open parentheses negative log 3 minus 1 third plus log space 4 close parentheses space minus space left parenthesis negative log space 1 space minus space 1 space plus space log space 2 right parenthesis
equals negative log 3 minus 1 third plus log space 4 space plus space 1 space minus log 2 equals 2 over 3 plus log open parentheses fraction numerator 4 over denominator 3 cross times 2 end fraction close parentheses space equals space 2 over 3 plus log 2 over 3

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