Q. The solution of the differential equation $\sec^{2}x\tan y\ dx + \sec^{2}y\tan x\ dy = 0$ is

A) $\tan y\tan x = c$
B) $\frac{\tan y}{\tan x} = c$
C) $\frac{\tan^{2}x}{\tan y} = c$
D) None of these
Explanation

(a)
Given, $\frac{\sec^{2}x}{\tan x}dx = - \frac{\sec^{2}y}{\tan y}dy$
$\Rightarrow$ $\int_{}^{}\frac{\sec^{2}x}{\tan x}dx = - \int_{}^{}\frac{\sec^{2}y}{\tan y}dy\$
Put $\tan x = u$
$\Rightarrow$ $\sec^{2}x\ dx = du$
And $\tan y = v$
$\Rightarrow$ $\sec^{2}y\ dy = dv$
$\therefore$ $\int_{}^{}\frac{du}{u} = - \int_{}^{}\frac{dv}{v}$
$\Rightarrow$ $\log u = - \log v + \log c \Rightarrow uv = c$
$\therefore$ $\tan x.\tan y = c$

Dr. Abubacker
Reviewed by Dr. Abubacker

Physics Educator & NEET/JEE Specialist

Ph.D. in Physics with 19+ years of teaching experience.