（4a-5b-ab）-（2a-3b+5ab)
=4a-5b-ab-2a+3b-5ab
=2a-2b-6ab
=2(a-b)-6ab
=2×2-6×(-1)
=4+6
=102a-2b-6ab怎么得到4a-5b-ab-2a+3b-5ab=4a-2a-5b+3b-ab-5ab=2a-2b-6ab