- #1
euro94
- 26
- 0
Homework Statement
cos(x-y)cosy-sin(x-y)siny=cosx
a.try to prove that the equation is an identity
b. determine a counterexample to show that it is not an identity
Homework Equations
cos(x-y) = cosxcosy+sinxsiny
sin(x-y) = sinxcosy-cosxsiny
The Attempt at a Solution
a.Left side of equatioin: (cosxcosy+sinxsiny)cosy - (sinxcosy-cosxsiny)siny
= cosxcosycos2y+sinxcosysinycosy - (sinxcosysiny - cosxsin2y)
I'm not sure where to go from there ...
b. how would i go about finding a counterexample?