3.计算 2( xdy,其中L为正向圆周(x-1)2+y2=2 4.证明下列曲线积分在整个xoy面内与路径无关,并计算积分值: (1)(6xy2-y3)dx+(6x2y-3xy2)dy; (1,2) (2,1) (2)(2xy-y4+3)dx+(x2-4xy3)d