cosx=(1-tan^2(x/2))/(1+tan^2(x/2))原式等于积[2(1+tan^2(x/2))/(4+2tan^2(x/2))]darctan(tan(x/2))等于积1/(2+tan^2(x/2))dtan(x/2)等于arctan((tan(x/2))/√2))/√2+C也就是
