a.maths

if A+B+C=180 ,

(a) prove that cos3A + cos3B+ cos3C= 1- 4 sin(3A/2) sin(3B/2) sin (3C/2)

(b) In triangle ABC, given that cos 3A+ cos3B + cos 3C=1, where A, B, C are the interior angles, show that one of the angles of the triangles is 120



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