In traingle ABC, cos A = sqrt(7/10) and cos B = 3/5. Find cos C.
arccos (7/10) ≈ 45.57°
arccos (3/10) ≈ 72.54°
C = ( 180 - 45.57 - 72.54) = 61.89°
cos 61.89° ≈ .47
