JEE MAIN - Mathematics (2024 - 31st January Evening Shift - No. 22)
Let $$y=y(x)$$ be the solution of the differential equation
$$\sec ^2 x d x+\left(e^{2 y} \tan ^2 x+\tan x\right) d y=0,0< x<\frac{\pi}{2}, y(\pi / 4)=0$$.
If $$y(\pi / 6)=\alpha$$, then $$e^{8 \alpha}$$ is equal to ____________.
$$\sec ^2 x d x+\left(e^{2 y} \tan ^2 x+\tan x\right) d y=0,0< x<\frac{\pi}{2}, y(\pi / 4)=0$$.
If $$y(\pi / 6)=\alpha$$, then $$e^{8 \alpha}$$ is equal to ____________.
Answer
9
Explanation
$$\begin{aligned}
& \sec ^2 x \frac{d x}{d y}+e^{2 y} \tan ^2 x+\tan x=0 \\
& \left(\text { Put } \tan x=t \Rightarrow \sec ^2 x \frac{d x}{d y}=\frac{d t}{d y}\right) \\
& \frac{d t}{d y}+e^{2 y} \times t^2+t=0 \\
& \frac{d t}{d y}+t=-t^2 \cdot e^{2 y} \\
& \frac{1}{t^2} \frac{d t}{d y}+\frac{1}{t}=-e^{2 y} \\
& \left(\text { Put } \frac{1}{t}=u \frac{-1}{t^2} \frac{d t}{d y}=\frac{d u}{d y}\right) \\
& \frac{-d u}{d y}+u=-e^{2 y} \\
& \frac{d u}{d y}-u=e^{2 y} \\
& \text { I.F. }=e^{-\int d y}=e^{-y} \\
& u e^{-y}=\int e^{-y} \times e^{2 y} d y \\
& \frac{1}{\tan x} \times e^{-y}=e^y+c \\
& x=\frac{\pi}{4}, y=0, c=0 \\
& \begin{aligned}
& \mathrm{x}=\frac{\pi}{6}, \quad \mathrm{y}=\alpha \\
& \sqrt{3} \mathrm{e}^{-\alpha}=\mathrm{e}^\alpha+0 \\
& \mathrm{e}^{2 \alpha}=\sqrt{3} \\
& \mathrm{e}^{8 \alpha}=9
\end{aligned}
\end{aligned}$$
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