MCQ
$\int_0^{\pi / 4} \frac{\sec ^2 x}{(1+\tan x)(2+\tan x)} d x=$
  • A
    $\log _e\left(\frac{2}{3}\right)$
  • B
    $\log _e 3$
  • C
    $\frac{1}{2} \log _e\left(\frac{4}{3}\right)$
  • $\log _e\left(\frac{4}{3}\right)$

Answer

Correct option: D.
$\log _e\left(\frac{4}{3}\right)$
(D)
Put $1+\tan x= t \Rightarrow \sec ^2 x d x= dt$
When $x=0, t =1$ and when $x=\frac{\pi}{4}, t =2$
$\therefore \quad \int_0^{\pi / 4} \frac{\sec ^2 x}{(1+\tan x)(2+\tan x)} d x$
$\begin{array}{l}=\int_1^2 \frac{d t}{t(1+t)}=\int_1^2 \frac{d t}{t}-\int_1^2 \frac{d t}{1+t} \\ =[\log t-\log (1+t)]_1^2\end{array}$
$\begin{array}{l}=\log _e 2-\log _e 3+\log _e 2 \\ =\log _e\left(\frac{4}{3}\right)\end{array}$

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