已知A=5a+3b,B=3a^2-2a^2b,c=a^2+7a^2b-2,当a=1,b=2时,求A-2B-3C的值
问题描述：
已知A=5a+3b,B=3a^2-2a^2b,c=a^2+7a^2b-2,当a=1,b=2时,求A-2B-3C的值
答
A-2B-3C
=(5a+3b)-2(3a^2-2a^2b)-3(a^2+7a^2b-2)
=5a+3b-6a^2+4a^2b-3a^2-21a^2b+6
=5a+3b-9a^2-17a^2b+6
=5+6-9-34+6
=-26