By regrouping the LHS and multiplying and dividing by 2 we get,

But 2cos A cos B = cos (A+B) +cos (A–B)

Then the above equation becomes,

But cos(90°–θ)=sin θ and cos(180°–θ)=–cos(θ).

Then the above equation becomes,

Now,

Substituting the corresponding values, we get

Hence proved