文档介绍：.:..(1)(3x-5y)-(6x+7y)+(9x-2y)(2)2a-[3b-5a-(3a-5b)](3)(6m2n-5mn2)-6(m2n-mn2):A=3x2-4xy+2y2,B=x2+2xy-5y2求:(1)A+B(2)A-B(3)若2A-B+C=0,求C。:(1)m2+(-mn)-n2+(-m2)-(-)(2)2(4an+2-an)-3an+(an+1-2an+1)-(8an+2+3an)(3)化简:(x-y)2-(x-y)2-[(x-y)2-(x-y)2]例4求3x2-2{x-5[x-3(x-2x2)-3(x2-2x)]-(x-1)}的值,其中x=2。-1y5和-x5y2n+1是同类项,求3m+2n的值。+y=6,xy=-4,求:(5x-4y-3xy)-(8x-y+2xy)的值。三、练****一)计算:(1)a-(a-3b+4c)+3(-c+2b)(2)(3x2-2xy+7)-(-4x2+5xy+6)(3)2x2-{-3x+6+[4x2-(2x2-3x+2)]}(二)化简(1)a>0,b<0,|6-5b|-|3a-2b|-|6b-1|(2)1<a<3,|1-a|+|3-a|+|a-5|(三)当a=1,b=-3,c=1时,求代数式a2b-[a2b-(5abc-a2c)]-5abc的值。(四)当代数式-(3x+6)2+2取得最大值时,求代数式5x-[-x2-(x+2)]的值。精品.(五)x2-3xy=-5,xy+y2=3,求x2-2xy+y2的值。1解:(1)(3x-5y)-(6x+7y)+(9x-2y)=3x-5y-6x-7y+9x-2y(正确去掉括号)=(3-6+9)x+(-5-7-2)y(合并同类项)=6x-14y(2)2a-[3b-5a-(3a-5b)](应按小括号,中括号,大括号的顺序逐层去括号)=2a-[3b-5a-3a+5b](先去小括号)=2a-[-8a+8b](及时合并同类项)=2a+8a-8b(去中括号)=10a-8b(3)(6m2n-5mn2)-6(m2n-mn2)(注意第二个括号前有因数6)=6m2n-5mn2-2m2n+3mn2(去括号与分配律同时进行)=(6-2)m2n+(-5+3)mn2(合并同类项)=4m2n-2mn22解:(1)A+B=(3x2-4xy+2y2)+(x2+2xy-5y2)=3x2-4xy+2y2+x2+2xy-5y2(去括号)=(3+1)x2+(-4+2)xy+(2-5)y2(合并同类项)=4x2-2xy-3y2(按x的降幂排列)(2)A-B=(3x2-4xy+2y2)-(x2+2xy-5y2)=3x2-4xy+2y2-x2-2xy+5y2(去括号)=(3-1)x2+(-4-2)xy+(2+5)y2(合并同类项)=2x2-6xy+7y2(按x的降幂排列)(3)∵2A-B+C=0∴C=-2A+B=-2(3x2-4xy+2y2)+(x2+2xy-5y2)=-6x2+8xy-4y2+x2+2xy-5y2(去括号,注意使用分配律)=(-6+1)x2+(8+2)xy+(-4-5)y2(合并同类项)=-5x2+10xy-9y2(按x的降幂排列)3解:(1)m