Evaluate : \(\int_1^2\frac{dx}{x^2+6x+5}\)
Q.2 (B) (iii) Evaluate : \(\int_1^2\frac{dx}{x^2+6x+5}\)
Q.2 (B) (iii) Evaluate : \(\int_1^2\frac{dx}{x^2+6x+5}\)
Best answer
Answer: \(\frac{1}{4}\log\frac{9}{7}\)
\(x^2+6x+5=(x+3)^2-2^2\). Using \(\int\frac{dx}{X^2-a^2}=\frac{1}{2a}\log\left|\frac{X-a}{X+a}\right|\):
\(I=\frac{1}{4}\left[\log\left|\frac{x+3-2}{x+3+2}\right|\right]_1^2=\frac{1}{4}\left[\log\frac{x+1}{x+5}\right]_1^2\)
\(=\frac{1}{4}\left[\log\frac{3}{7}-\log\frac{2}{6}\right]=\frac{1}{4}\log\left(\frac{3}{7}\times3\right)=\frac{1}{4}\log\frac{9}{7}\)
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