Find , when:

If cos y = x cos (y + a), prove that .

Let us differentiate the whole equation w.r.t x

Formula :

According to chain rule of differentiation

According to product rule of differentiation

Therefore ,

( Multiply and divide by cos (y+a) )

(Since cos y = x cos (y + a))

(Formula sin(a-b)=sin a cos b – cos a sin b)