先化简,再求值:(x+y)(2x-y)-(x+2y)(x-2y),其中x=3,y=4.
人气:134 ℃ 时间:2020-02-06 05:00:55
解答
(x+y)(2x-y)-(x+2y)(x-2y)
=2x2-xy+2xy-y2-(x2-4y2)
=2x2-xy+2xy-y2-x2+4y2
=x2+xy+3y2;
把x=3,y=4代入得,
原式=32+3×4+3×42
=9+12+3×16
=21+48
=69.
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