Question

cos 5x cos 2x+sin 5x sin 2x

Answer 1

cos (α — β) = cos α cos β + sin α sin β. 

cos (5x - 2x) = cos (3x) 

Answer 2

cos(a-b)=cosa×cosb+sina×sinb
cos 5x cos 2x+sin 5x sin 2x=cos(5x-2x)=cos3x