Question

先化简,再求值5ab-[2ab²-3(ab²-ab+1)]-3a ,其中b=-3

Answer 1

原式
=5ab-[2ab平方-3ab平方+3ab-3]-3a
=5ab+ab平方-3ab+3-3a
=ab平方+2ab-3a+3

追问

但是这样a消不掉

追答

=9a-6a-3a+3
=3

代入b，就将a消掉了

Answer 2

5ab-[2ab²-3(ab²-ab+1)]-3a
=5ab-[2ab²-3ab²+3ab-3]-3a
=5ab-[-ab²+3ab-3]-3a
=5ab+ab²-3ab+3-3a
=2ab+ab²+3-3a
=a(2b+b²-3)+3
代入b=-3，值为3