Ta có:
f (-1) = (2-a)(-1)2 + 5a(-1) - 7 = 2 - a - 5a - 7 = - 6a - 5
f(2) = (2-a)22 + 5a.2 - 7 = 8 - 4a + 10a - 7 = 6a + 1
f(-1) = f(2) => - 6a - 5 = 6a + 1
<=> 12a = - 6 => a = - 1/2
Ta có:
f(-1)=(2-a)*(-1)2+5a*(-1)-7
=2-a-5a-7
=-5-6a
f(2)=(2-a)*22+5a*2-7
=(2-a)*4+10a-7
=8-4a+10a-7
=6a+1
Mà f(-1)=f(2). Suy ra -5-6a=6a+1
Suy ra 12a=-6
a=-1/2
Vậy a=-1/2