Prove that:
By regrouping the LHS and multiplying and dividing by 4 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