(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 $\tan y = v$
$\Rightarrow$ $\sec^{2}y\ dy = dv$
$\therefore$ $\int_{}^{}\frac{du}{u} = - \int_{}^{}\frac{dv}{v}$
$\Rightarrow$ $\log u = - \log v + \log c \Rightarrow uv = c$
$\therefore$ $\tan x.\tan y = c$