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

Here,

cos y = x cos (a + y), where cos a

Differentiating both sides with respect to x, we get

Multiplying the numerator and the denominator by cos(a+y) on th RHS we have,

[Given cos y = x cos (a + y)]

[ sin(a-b)=sina cosb - cosa sinb]

Hence Proved.