Question

3√2*cos²*9π8 - 3√2*sin²*9π8

Answer 1

$3 sqrt{2} cos^2 frac{9 pi }{8}- 3 sqrt{2}sin^2 frac{9 pi }{8}=\ 3 sqrt{2}(cos^2 frac{9 pi }{8}- sin^2 frac{9 pi }{8})=3 sqrt{2}cosfrac{18 pi }{8}=3 sqrt{2}cosfrac{9pi }{4}=3 sqrt{2}cos(2 pi +frac{pi }{4})=\3 sqrt{2}cosfrac{pi }{4}=3 sqrt{2} frac{ sqrt{2} }{2} =3$