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.
