题目内容

计算与化简
(1)
5
8
×(-4)2-0.25×(-5)÷(-
1
2
3
(2)当x=-
1
2
,y=-3时,求代数式3(x2-2xy)-[3x2-2y+2(xy+y)]的值.
(1)
5
8
×(-4)2-0.25×(-5)÷(-
1
2
3
=
5
8
×16-0.25×(-5)÷(-
1
8

=10-0.25×5×8
=10-10
=0;

(2)3(x2-2xy)-[3x2-2y+2(xy+y)]
=3x2-6xy-[3x2-2y+2xy+2y]
=3x2-6xy-3x2-2xy
=-8xy;
当x=-
1
2
,y=-3时,
-8xy=-8×(-
1
2
)×(-3)=-12.
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