tan(arctan1/2+arctan1/春码誉3)
=[tan(arctan(1/2))+tan(arctan(1/3))]/[1-tan(arctan(1/2))*tan(arctan(1/3))
=(1/2+1/3)/(1-1/2*1/模姿3)=1
arctan1/2+arctan1/3=pi/4
x=sint,-pi/扒段2<=t<=pi/2
arcsinx=arcsin(sint)=t
sqrt(1-x^2)=sqrt(1-(sint)^2)=sqrt((cost)^2)=cost=cos(arcsinx)
搜一下：孙孙求助高数题:证明恒等式则袜链arctan1/好段2+arctan1/3=45度
cos(arcsinx)=根号(1-x^2)在线急等！