已知sin4次方-cos4次方=5/9,求cos4
问题描述：
已知sin4次方-cos4次方=5/9,求cos4
答
sin4次方-cos4次方=5/9(sin2次方+cos2次方)(sin2次方-cos2次方)=5/91 * (sin2次方-cos2次方)=5/9-cos(2x)=5/9cos(2x) = -5/9cos(4x) = 2{cos(2x)}^2 - 1 = 2 * (-5/9)^2 - 1 = -31/81