利用极坐标x= rcos t,y= rain 4 2 dtr 2 cost sint r21+4r2rdr 2[sin[r5、1+4r2dr令u=1+4r2 L 125√5-1 420利用极坐标 x = r cost , y = rsint , dt r t t r r rdr   =  + 1 0 2 2 2 2 0 4 cos sin 1 4  tdt r r dr 2 1 0 5 0 2 sin2 1 4 2 = +    令 2 u = 1+ 4r du u u 2 5 1 ) 4 1 ( 4 1 − =  . 420 125 5 −1 =