6.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) (int )_((1,1))^(-2)(x+y)dx+(x-y)dy;-|||-(2)∫ (1,2)(6x(y)^2-(y)^3)dx+(6(x)^2y-3x(y)^2)dy;-|||-(3) (int )_((1.0))^(2xy)(2xy-(y)^4+3)dx+((x)^2-4x(y)^3)dy.