$\int\left[\frac{(1+\log x)}{\cos ^{2}(x \log x)}\right] d x=$

$\int\left[\frac{(1+\log x)}{\cos ^{2}(x \log x)}\right] d x=$
  1. $\sin (x \log x)+c$
  2. $\sin ^{2}(x \log x)+c$
  3. $\log (x \log x)+c$
  4. $\tan (x \log x)+c$

Solution

$I=\int \frac{(1+\log x)}{\cos ^{2}(x \log x)} d x$ Put $x \log x=t \Rightarrow\left[x \cdot \frac{1}{x}+\log x(1)\right] d x=d t \Rightarrow(1-\log x) d x=d t$ $1=\int \frac{1}{\cos ^{2} t} d t=\int \sec ^{2} t d t=\tan t+c$ $=\tan (x \log x)-c$

Asked in: MHT CET 2020 (13 Oct Shift 2)

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