万能公式为：设 tan X = Y,则 sin(2X) = 2Y/(1+Y^2)(sinx-2cosx)/(sinx+3cosx)=2 对于左端,分子\分母同时除以 cosx(tanx -2)/(tanx + 3) = 2tanx -2 = 2(tanx + 3)tanx = -8sinx*cosx = (1/2) * sin(2x) = (1/2) * 2...