Question

解下面这道题,已知3(ax²+2x-1)-(9x²+6x-7)的值与x无关 求 5a²-2(a²-3a+4)的值

Answer 1

解：
3(ax²+2x-1)-(9x²+6x-7)
=3ax²+6x-3-9x²-6x+7
=(3ax²-9x²)+(6x-6x)+(-3+7)
=(3a-9)x²+4
代数式的值
与x无关，说明(3a-9)x²的系数(3a-9)为0
3a-9=0,
a=3
5a²-2(a²-3a+4)
=5a²-2a²+6a-8
=3a²+6a-8
=3×3²+6×3-8
=27+18-8
=37