Functions of Compound Angles

1.If A, B and C are acute angles and cot A = 2 , cot B = 5 and cotC = 8 , find (A + B + C) without using calculators.


2. In ㄥABC , Suppose that sin A = sin(B + C) and tan B = cos(B - C) / [sin A - sin(B - C)]
(i) show that cos Bcos(B +C) = 0 ,
(ii) hence, prove that A=兀/2