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posted by Brandon Tuesday, November 24, 2009 at 11:06am.
Use a counter example to show that cos(x+y)= cosx +cosy is not an identity
Let x=π/4, y=π/4, cos(x+y) =cos(π/4+π/4) =cos(π/2) =0 cos(x)+cos(y) =cos(π/4)+cos(π/4) =√2/2+√2/2 =√2 Since √2 ≠ 0, cos(x+y) ≠ cosx +cosy
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