例1 Va2-x2dx.x=aint dx=acos tdt. va2-x a cos t (x:0~a,t:0~) 2 sin 2t =a2|2 2 cos tdt 2(1+cos 2t dt=-(t+ 0 2◼ 例1 2 2 2 2 0 2 2 2 2 2 2 2 0 0 0 2 , sin , cos , cos ( : 0 , : 0 ) 2 sin 2 cos (1 cos 2 ) ( ) 2 2 2 4 a t a x dx x a t dx a tdt a x a t x a t a a t a tdt t dt t a      = − = = − =  = = + = + =  