If $\mathrm{I}=\int \sin (\log (x)) \mathrm{d} x$, then $\mathrm{I}$ is given by
If $\mathrm{I}=\int \sin (\log (x)) \mathrm{d} x$, then $\mathrm{I}$ is given by
- $-\frac{x}{2}(\sin (\log x)-\cos (\log x))+\mathrm{c}$, where $\mathrm{c}$ is a constant of integration.
- $\frac{x}{2}(\sin (\log x)-\cos (\log x))+\mathrm{c}, \text { where } \mathrm{c}$
is a constant of integration.
- $\frac{x}{2}(\sin (\log x)+\cos (\log x))+\mathrm{c}$, where $\mathrm{c}$ is a constant of 'integration.
- $-\frac{x}{2}(\sin (\log x)+\cos (\log x))+\mathrm{c}$, where c is a constant of integration.
Solution
Let $\log x=\mathrm{t} \Rightarrow x=\mathrm{e}^{\mathrm{t}}$ Differentiating w.r.t. t, we get $\mathrm{d} x=\mathrm{e}^{\mathrm{t}} \mathrm{dt}$
$\begin{aligned}
\therefore \quad \mathrm{I} & =\int \sin (\mathrm{t}) \mathrm{e}^{\mathrm{t}} \mathrm{dt} \\
& =\sin \mathrm{t} \mathrm{e}^{\mathrm{t}}-\int \cos \mathrm{t} \mathrm{e}^{\mathrm{t}} \mathrm{dt} \\
& =\sin \mathrm{t} \mathrm{e}^{\mathrm{t}}-\left[\cos \mathrm{t} \mathrm{e}^{\mathrm{t}}+\int \sin \mathrm{t} \mathrm{e}^{\mathrm{t}} \mathrm{dt}\right] \\
& =\sin \mathrm{t} \mathrm{e}^{\mathrm{t}}-\cos \mathrm{t} \mathrm{e}^{\mathrm{t}}-\mathrm{I}
\end{aligned}$
$\begin{array}{ll}\therefore & 2 \mathrm{I}=x \sin (\log x)-x \cos (\log x)+\mathrm{c} \\ \therefore \quad & \mathrm{I}=\frac{x}{2}(\sin (\log x)-\cos (\log x))+\mathrm{c}\end{array}$
Asked in: MHT CET 2023 (11 May Shift 2)
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