cos（π/2+Q）=1/3 着 cos（π-2Q）=?
问题描述：
cos（π/2+Q）=1/3 着 cos（π-2Q）=?
答
cos(π/2+Q) = cos(-π/2-Q) = -cos(π/2-Q) = -1/3
cos(π-2Q) = 2cos^2(π/2-Q)-1 = 2 * (-1/3)^2 -1 = -7/9