Jawapan

2016-06-13T20:29:53+08:00

f(x)=-(x²-4x-a²)                          

     =-[x²-4x+(-2)²-(-2)²-a²]

     =-[(x-2)²-4-a²]

     =-(x-2)²+4+a²

4+a²=8

a²=4

a=2,-2