求值(1)先化简,再求值:(x2y3-2x3y2)÷(-12xy2)-[2(x-y)]2,其中x=3,y=-12.(2)已知a+b=3,ab=-2.求ab-a2-b2的值.-数学

题目简介

求值(1)先化简,再求值:(x2y3-2x3y2)÷(-12xy2)-[2(x-y)]2,其中x=3,y=-12.(2)已知a+b=3,ab=-2.求ab-a2-b2的值.-数学

题目详情

求值
(1)先化简,再求值:(x2y3-2x3y2)÷(-
1
2
xy2)-[2(x-y)]2,其中x=3,y=-
1
2

(2)已知a+b=3,ab=-2.求ab-a2-b2的值.
题型:解答题难度:中档来源:不详

答案

(1)原式=-2xy+4x2-4x2+8xy-4y2=6xy-4y2.
 当x=3,y=-class="stub"1
2
时,
原式=6×3×(-class="stub"1
2
)-4×(-class="stub"1
2
)2
=-9-1=-10.

(2)当a+b=3,ab=-2时,
  ab-a2-b2
=-(a+b)2+3ab
=-32+3×(-2)
=-15.

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