Question

已知A=5a 3b,B=3a的平方-2a的平方b,C=a的平方 7a的平方d-2,当a=1,b=2时,求A-2B 3C的

Answer 1

A一2B＋3C

=5a十3b-2(3a^2一2a^2b)+3(a^2＋7a^2b一2)

=5a+3b-6a^2+4a^2b+3a^2+21a^2b-6

=25a^2b-3a^2+5a+3b-6

将a=l,b=2代入上式得：

25*1^2*2-3*1^2+5*1+3*2-6

=50-3+5+6-6

=52