一、选择题(每小题4分,共12分)
1.下列哪个式子的计算结果为7a2-7ab ( )
A.(3a2-ab+7)-(4a2+6ab+7)
B.(3a2-ab+7)-(-4a2-6ab+7)
C.(3a2-ab+7)-(-4a2-6ab-7)
D.(3a2-ab+7)-(-4a2+6ab+7)
【解析】选D.因为(3a2-ab+7)-(4a2+6ab+7)=3a2-ab+7-4a2-6ab-7=-a2-7ab;
(3a2-ab+7)-(-4a2-6ab+7)
=3a2-ab+7+4a2+6ab-7=7a2+5ab;
(3a2-ab+7)-(-4a2-6ab-7)
=3a2-ab+7+4a2+6ab+7
=7a2+5ab+14;
(3a2-ab+7)-(-4a2+6ab+7)
=3a2-ab+7+4a2-6ab-7
=7a2-7ab.
【变式训练】若A=x2-2xy+y2,B=x2+2xy+y2,则下列各式运算结果等于4xy的是
( )
A.A+B B.A-B
C.-A+B D.-A-B
【解析】选C.因为A=x2-2xy+y2,B=x2+2xy+y2,
所以-A=-x2+2xy-y2,
所以-A+B=-x2+2xy-y2+x2+2xy+y2=4xy,
所以运算结果等于4xy的是-A+B.
