/x\ /y\ /x + y\ cos|-|*cos|-|*cos|-----| \2/ \2/ \ 2 /
/x\ /y\ /x y\ cos|-|*cos|-|*cos|- + -| \2/ \2/ \2 2/
cos(x/2)*cos(y/2)*cos((x + y)/2)
1 cos(x) cos(y) cos(x + y) - + ------ + ------ + ---------- 4 4 4 4