using your hint the product identity is
using the product identiy again (twice) we end up with
now we use the fact that and
sub these into the above and use the sum identity for cosine and you should be done.
We got this question the other day and I'm not exactly sure how to do it.
Given A+B+C= pi
cos(2A)+cos(2B)+cos(2C)+1 = -4cos(A)cos(B)cos(C)
The hint was to use product formulas.
Can anybody help me with how I would go about doing this?