Let $y=y(x)$ be a solution of the differential equation $(x \cos x) d y+(x y \sin x+y \cos x-1) d x=0,0 < x < \frac{\pi}{2}$. If $\frac{\pi}{3} y\left(\frac{\pi}{3}\right)=\sqrt{3}$, then $$\left|\frac{\pi}{6} y^{\prime \prime}\left(\frac{\pi}{6}\right)+2 y^{\prime}\left(\frac{\pi}{6}\right)\right|$$ is equal to ____________.
Answer (integer)
2
Solution
Given, differential equation
<br/><br/>$$
\begin{aligned}
& (x \cos x) d y+(x y \sin x+y \cos x-1) d x=0,0 < x < \frac{\pi}{2} \\\\
& \Rightarrow \frac{d y}{d x}+\frac{x y \sin x+y \cos x-1}{x \cos x}=0 \\\\
& \Rightarrow \frac{d y}{d x}+\left(\frac{x y \sin x+y \cos x}{x \cos x}\right)=\frac{1}{x \cos x} \\\\
& \Rightarrow \frac{d y}{d x}+\left(\tan x+\frac{1}{x}\right) y=\frac{1}{x \cos x}
\end{aligned}
$$
<br/><br/>Which is linear differential equation in the form of
<br/><br/>$$
\begin{aligned}
& \frac{d y}{d x}+P y=Q \\\\
& \therefore \mathrm{IF}=e^{\int\left(\tan x+\frac{1}{x}\right) d x}=e^{(\log \sec x+\log x)}=e^{\log (x \sec x)}=x \sec x
\end{aligned}
$$
<br/><br/>$\therefore$ The general solution of the given differential equation
<br/><br/>$$
\begin{array}{rlrl}
y \cdot \mathrm{IF} =\int(Q \times \mathrm{IF}) d x+c \\\\
\Rightarrow y(x \sec x) =\int\left(\frac{1}{x \cos x} x \sec x\right) d x+c \\\\
\Rightarrow x y \sec x =\int \sec ^2 x d x+c \\\\
\Rightarrow x y \sec x =\tan x+c ........(i)
\end{array}
$$
<br/><br/>Since, $\frac{\pi}{3} y\left(\frac{\pi}{3}\right)=\sqrt{3} \Rightarrow y\left(\frac{\pi}{3}\right)=\frac{3 \sqrt{3}}{\pi}$
<br/><br/>$$
\begin{aligned}
& \therefore \frac{\pi}{3}\left(\frac{3 \sqrt{3}}{\pi}\right) \sec \left(\frac{\pi}{3}\right)=\tan \left(\frac{\pi}{3}\right)+c \\\\
& \Rightarrow \sqrt{3}(2)=\sqrt{3}+c \\\\
& \begin{aligned}
\Rightarrow & c=\sqrt{3}
\end{aligned}
\end{aligned}
$$
<br/><br/>On putting the value of $c$ in Eq. (i), we get
<br/><br/>$$
\begin{aligned}
&x y \sec x =\tan x+\sqrt{3} \\\\
&\Rightarrow y =\frac{1}{x}(\sin x+\sqrt{3} \cos x) \\\\
& y^{\prime} =\frac{1}{x}(\cos x-\sqrt{3} \sin x)-\frac{1}{x^2}(\sin x+\sqrt{3} \cos x)
\end{aligned}
$$
<br/><br/>$\begin{aligned} & \text { and } y^{\prime \prime}=\frac{1}{x}(-\sin x-\sqrt{3} \cos x)-\frac{1}{x^2}(\cos x-\sqrt{3} \sin x) -\frac{1}{x^2}(\cos x-\sqrt{3} \sin x) \\ & -\frac{2}{x^3}(\cos x-\sqrt{3} \sin x) \\\\ & \therefore\left|\frac{\pi}{6} y^{\prime \prime}\left(\frac{\pi}{6}\right)+2 y^{\prime}\left(\frac{\pi}{6}\right)\right|=|-2|=2 \\\\ & \end{aligned}$
About this question
Subject: Mathematics · Chapter: Differential Equations · Topic: Order and Degree
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