I don´t know how to explane it. When i put in Mathematica a and b 45deg (a=b=45deg)
the function is plot (on x axis there´s c and on y axis amplitude) periodical from 0 to 1
in amplitude. Any other combination of a and b for example a=b=20deg will also achive
maximum at 1 but minimum would be in 0.6. The same is if the anyone of a or b is negative.
All this results are good. But if the a and b are not equal the ploted function would not have maximum at 1 but less then 1 and this solution is no good.
The question is how can i calculate from the 1/4(2+Cos[2(a-b)]+Cos[2(a+b)]-2Cos[c]Sin[2a]Sin[2b] a and b teoreticaly because upper results are only practical.
The real solution must be quide easy but i don´t see it.