Question
Evaluate the integral $\int_{0}^{1} \frac{x}{x^{2}+1} d x$ using substitution.

Answer

Given integral is: $\int_{0}^{1} \frac{x}{x^{2}+1} d x$ 
Let x2 + 1 = t
⇒ 2xdx = dt
⇒ xdx = $\frac{1}{2}$ dt
When x = 0, t = 1 and when x = 1, t = 2
$\Rightarrow \int_{0}^{1} \frac{x}{x^{2}+1} d x=\int_{1}^{2} \frac{d t}{2 t}$ 
$=\frac{1}{2} \int_{1}^{2} \frac{d t}{t}$ 
$=\frac{1}{2}\left[\log |t| ]_{1}^{2}\right.$ 
= $\frac{1}{2}[\log 2-\log 1]$ 
= $\frac{1}{2} \log 2$

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