设x=3cosu,y=4sinu,ds=√[(dx)^2+(dy)^2]=√[9(cosu)^2+16(sinu)^2]du=√[(9/2)(1+cos2u)+8(1-cos2u)]du=√(25-7cos2u)du/√2，∮24xyds=∫<0,2π>144sin2u√(25-7cos2u)du/√2=（-24√2/7）（25-7cos2u）^(3/2)|<0,2π>=0
