Question

Prove that cos(45° + theta) · cos(45° - theta) = 5 cos 2theta​

Answer 1

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Answer 2

Step-by-step explanation:

LHS

cos(45+A).cos(45-A)

=(cos45cosA-sin45sinA).(cos45cosA+ sin45sinA)

=cos²45cos²A-sin²45sin²A

=1/2(cos²A-sin²A)

=1/2.cos2A