(a) Find $\frac{d y}{d x}$, , if $(\cos x)^y=(\cos y)^x$.

OR

(b) If $\sqrt{1-x^2}+\sqrt{1-y^2}=a(x-y)$, prove that $\frac{d y}{d x}=\sqrt{\frac{1-y^2}{1-x^2}}$.

(a) Differentiate $$\quad \sec ^{-1}\left(\frac{1}{\sqrt{1-x^2}}\right) \quad$$ w.r.t. $$\sin ^{-1}\left(2 x \sqrt{1-x^2}\right)$$.

OR

(b) If $$y=\tan x+\sec x$$, then prove that $$\frac{d^2 y}{d x^2}=\frac{\cos x}{(1-\sin x)^2}$$.