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

## Answer this Question

## Related Questions

- Math - Use a counter example to show that cos(x+y)= cosx +cosy is not an ...
- Math - Suppose f(x) = sin(pi*cosx) On any interval where the inverse function y...
- Trig - prove the identity (sinX)^6 +(cosX)^6= 1 - 3(sinX)^2 (cosX)^2 sinX^6= ...
- Trigonometry - Write equivalent equations in the form of inverse functions for a...
- math - sinx+ siny/ (cosx+cosy)= tan 1/2 (x+y) prove this identity
- Math help again - cos(3π/4+x) + sin (3π/4 -x) = 0 = cos(3π/4)cosx...
- Math - Verify the identity . (cscX-cotX)^2=1-cosX/1+cosX _______ sorry i cant ...
- Math - Pre- Clac - Prove that each of these equations is an identity. A) (1 + ...
- Pre-Calc - Trigonometric Identities Prove: (tanx + secx -1)/(tanx - secx + 1)= ...
- Geometry - Find 2 cos(x/2) sin(x/2) when x = -π/6. I know that cos(x/2...

More Related Questions