cos10 cos30 cos50 cos70 = cos10 cos70 cos50 cos30 = 1/2(cos80 + cos60) cos50 cos30 = 1/2(cos80 cos50 + cos60 cos50) cos30 = 1/2[(cos130 + cos30)/2 + cos50 /2)] cos30 = 1/4[- cos50 + cos30 + cos50] cos...