13. (2008山东烟台)已知,求的值.
12.(2008福建南平)先化简,再求值:,其中,.
解:原式
当,时,原式
11.(2008 江西南昌), 其中.
解:原式=x2+2x-(x2-1)…………2分
= x2+2x-x2+1 …………3分
=2x+1 …………4分
当时,原式=2×()+1=0. …………6分
10.(2008江苏淮安)先化简,再求值:其中x=-1,y=.
解:原式=
当x=-1,y=时
原式=
9.(2008福建省泉州市)先化简下面的代数式,再求值:
, 其中。
解:5
8.(2008山东泰安)(1)(2008山东泰安)先化简,再求值:,其中.
解:原式=
=………………2分
=x-4 ………………3分
当时, 原式=………………4分
7.(2008湖南株洲)已知,求代数式的值.
解:原式=…=
由得,
代入原式=2
6.(2008 江西), 其中.
解:
······································································································ 2分
········································································································· 3分
.···················································································································· 4分
当时,原式.······································································ 6分
5.(2008 湖北 荆门)给出三个多项式X =2a2+3ab+b2,Y =3a2+3ab,Z = a2+ab,请你任选两个进行加(或减)法运算,再将结果分解因式.
解:(以下给出三种选择方案,其他方案从略)
解答一:Y + Z =(3a2+3ab)+ (a2+ab)
=4a2+4ab
=4a(a+b).
解答二: X- Z = (2a2+3ab+b2)-(a2+ab)
=a2+2ab+b2 =(a+b)2.
解答三:Y- X=(3a2+3ab)- (2a2+3ab+b2)
=a2- b2 =(a+b)(a-b).
4.(2008江苏南京)先化简,在求值:(2a+1)2-2(2a+1)+3,其中a=.
解:原式=4a2+4a+1-4a-2+3……………………………………………………3分
=4a2+2.…………………………………………………………………4分
当a=时,4a2+2=4×()2+2=10. …………………………………6分
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