Question

已知A=2x^2+3ax-2x-1,B=-x^2+ax-1,且3A+6B的值于x...

已知A=2x^2+3ax-2x-1,B=-x^2+ax-1,且3A+6B的值于x无关,求代数式-5a+2009的值.

Answer 1

A=2x^2+3ax-2x-1,B=-x^2+ax-1,则
3A+6B
=3(2x^2+3ax-2x-1)+6(-x^2+ax-1)
=6x^2+9ax-6x-3-6x^2+6ax-6
=15ax-6x-9
=x(15a-6)-9
解得的值不含X,那么15a-b=0,则a=2/5
将a值代入代数式中,得到-5a+2009=2007