prove that cos3tita=4cos cube tita -3cos tita
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Answer: Re write cos3tita= cos(2tita+tita)
cos3tita= cos2tita × costita - sin2tita × sin tita
= (2 cos ^2tita -1) × cos tita - 2 sin tita× costita×sintita
=(2 cos^2tita -1) × cos tita - 2 cos tita (1-cos^2tita)
=2 cos^3tita -cos tita- 2cos tita + 2cos^3 tita
=4 cos^3tita - 3 cos tita
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