用万能代换。设tgx/2=u则 dx=[2/(1+u^2)]ducosx=(1-u^2)/(1+u^2)代入1/(3+cosx)的du/(u^2+2)其原函数为(1/√2)*arctg(u/√2)把tgx/2=u代入得原函数为(1/√2)*arctg[(tgx/2)/√2)]