Practicing Success
The solution of the differential equation $\frac{d y}{d x}+\frac{1}{2} y \sec x=\frac{\tan x}{2 y}$, where $0 \leq x<\frac{\pi}{2}$ and $y(0)=1$, is given by |
$y^2=1-\frac{x}{\sec x+\tan x}$ $y^2=1+\frac{x}{\sec x+\tan x}$ $y=1+\frac{x}{\sec x+\tan x}$ $y=1-\frac{x}{\sec x+\tan x}$ |
$y^2=1-\frac{x}{\sec x+\tan x}$ |
We have, $\frac{d y}{d x}+\frac{1}{2} y \sec x=\frac{\tan x}{2 y}$ $\Rightarrow 2 y \frac{d y}{d x}+y^2 \sec x=\tan x$ $\Rightarrow \frac{d u}{d x}+(\sec x) u=\tan x$, where $ u=y^2$ This is a linear differential equation with Integrating factor = $e^{\int \sec x d x}=e^{\log (\sec x+\tan x)}=\sec x+\tan x$ Multiplying both sides by integrating factor $=\sec x+\tan x$ and integrating, we get $u(\sec x+\tan x)=\int \tan x(\sec x+\tan x) d x+C$ or, $y^2(\sec x+\tan x)=\sec x+\tan x-x+C$ .........(i) It is given $y=1$ when $x=0$. Putting $x=0, y=1$ in (i), we get $C=0$. Putting $C=0$ in (i), we obtain $y^2(\sec x+\tan x)=\sec x+\tan x-x$ $\Rightarrow y^2=1-\frac{x}{\sec x+\tan x}$ |