已知A=5a十3b,B=3a的平方一2a的平方b,C=a的平方＋7a的平方b一2,当a=l,b=2时

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