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练一练: 1.计算: (1)(5a+b)+6a; (2)(2x-7)-(4x-5); (3)(3a2-3a-1)-(5a2+6a-8); (4)(8mn-3m2)-5mn-2(3mn-2m2). 解:(1)(5a+b)+6a =5a+b+6a =11a+b,(2)( 2x-7)-(4x-5) =2x-7-4x+5 =-2x-2,(3)( 3a2-3a-1)-(5a2+6a-8) =3a2-3a-1-5a2-6a+8 =-2a2-9a+7,(4)( 8mn-3m2)-5mn-2(3mn-2m2) =8mn-3m2-5mn-6mn+4m2 =m2-3mn,练一练: 2.求3y2-x2+(2x-y)-(x2+3y2)的值,其中x=1、 y=-2. 解:3y2-x2+(2x-y)-(x2+3y2) =3y2-x2+2x-y-x2-3y2 =(-1-1)x2+(3-3)y2+2x-y =-2x2+2x-y 将x=1、y=-2代入式中得到代数式的值是2.,