Prove that cos(45° + theta) · cos(45° - theta) = 5 cos 2theta
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Step-by-step explanation:
LHS
cos(45+A).cos(45-A)
=(cos45cosA-sin45sinA).(cos45cosA+ sin45sinA)
=cos²45cos²A-sin²45sin²A
=1/2(cos²A-sin²A)
=1/2.cos2A
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