Question

if sinA=1/2, then 4coscubeA-3cosA

Answer 1

cos (3A) = cos(2A + A),  = cos (2A) cos (A) - sin(2A) sin(A), = [ 2cos^2(A) - 1 ] cos (A) - (2 sin A cos A )sin A, = 2cos^3(A) - cos A - 2sin^2(A) cos A , = 2cos^3(A) - cos A - 2( 1 - cos^2(A)) cos A , = 2cos^3(A) - cos A - 2cos A + 2cos^3(A), = 4cos^3(A) - 3cos A