化简:
(1)(3a-2b)+(5a-7b)-2(2a-4b);
(2)(x3-xy2+x2y-y3)+(-y3+xy2-x2y-y3);
(3)(-x2+2xy-y2)-2(xy-3x2)+3(2y2-xy)。
解:(1)原式=3a-2b+5a-7b-4a+8b
=3a+5a-4a-2b-7b+8b=4a-b;
(2)原式=-
=++-2;
(3)原式=-x2+2xy-y2-2xy+6x2 +6y-3xy
=-x2+6x2+2xy-2xy-3xy-y2+6y2
=5x2-3xy+5y2。
当x=3,y=-时,求代数式3x2y-[ 2xy2-2(xy-x2y)+xy]+3xy2的值。
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