We are given with an equation cosy = x cos(a + y) , we have to prove that by using the given equation we will first find the value of and we will put this in the equation we have to prove, so by differentiating the equation on both sides with respect to x, we get,

– siny = cos(a + y) – x sin(a + y)

[xsin(a + y) – siny] = cos(a + y)

We can further solve it by using the given equation,

By using sinA cosB – cosA sinB = sin(A – B)