I'm having problems in proving that cos(θ+φ) cos(θ-φ) = cos²θ-sin²θ using the trigonometric identities. i'm having cos²θ-sin²φ as the answer. help!
... from previous poster. Continue as follows:
= (1 - sin²A).(1-sin²B) - sin²A.sin²B (sorry can't use greek letters !)
= 1 -sin²A - sin²B + sin²A.sin²B - sin²A.sin²B
= 1 -sin²A - sin²B
= sin²A + cos²A - sin²A - sin²B
= cos²A - sin²B
The question as written asked for the RHS to be cos²A - sin²A which is, of course, incorrect !