1. The problem statement, all variables and given/known data Use Euler's identity to prove that cos(u)cos(v)=(1/2)[cos(u-v)+cos(u+v)] and sin(u)cos(v)=(1/2)[sin(u+v)+sin(u-v)] 2. Relevant equations eui=cos(u) + isin(u) e-ui=cos(u)-isin(u) 3. The attempt at a solution I was able to this with other trig identities with no problem but this one I have hit a wall. we are supposed to start with e(u+v)i+e(u-v)i=eu(evi+e-vi) which becomes. cos(u+v)+isin(u+v)+cos(u-v)+isin(u-v)=eu(cos(v)+isin(v)+cos(v)-isin(v)) then equating the real parts cos(u+v)+cos(u-v)=eu(2cos(v)) then divide by 2 (1/2)[cos(u+v)+cos(u-v)]=eu(cos(v)) I cannot figure out why I have an eu and not a cos(u). Does anyone see where I have gone wrong or what I am missing? Thank you in advance.