Question 596221
cos(x-y)+cos(x+y)=2cos(x)cos(y)


[cos(x)cos(y)+sin(x)sin(y)]+[cos(x)cos(y)-sin(x)sin(y)]=2cos(x)cos(y)


cos(x)cos(y)+sin(x)sin(y)+cos(x)cos(y)-sin(x)sin(y)=2cos(x)cos(y)


[cos(x)cos(y)+cos(x)cos(y)]+[sin(x)sin(y)-sin(x)sin(y)]=2cos(x)cos(y)


2cos(x)cos(y)=2cos(x)cos(y)



This verifies the identity.