#### miran wrote:

1/4(2+Cos[2(a-b)]+Cos[2(a+b)]-2Cos[c]Sin[a]Sin)

I need to know what the a and b should be that this function c would be maximum.

If you want c to be a maximum, and c is in the function, then we are missing something ... perhaps this function is equal to a constant?

Also if it is cos(c), then c could take on many values, as ryos mentioned already. For example (using radians) cos(1)=0.540, and cos(7.28)=0.540, etc, each time adding 2*pi.

You can also use Excel to plot the function to see how it behaves, and also try different a and b values. This may not get you an exact answer, but could help to point the way.

May I ask what this is related to?