Posted by **Brandon** on Tuesday, November 24, 2009 at 11:06am.

Use a counter example to show that cos(x+y)= cosx +cosy is not an identity

- Math -
**MathMate**, Tuesday, November 24, 2009 at 3:44pm
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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