f(x) = x^+(a^-9)x+a^-5a+6 是开口向上的抛物线
f(x) = 0 有一根小于0另一根大于2,则
f(0) < 0
f(2) < 0
0 + (a^ - 9)*0 + a^ -5a + 6 < 0
2^ + (a^ -9)*2 + a^ - 5a + 6 < 0
a^ - 5a + 6 < 0
3a^ -5a - 12 < 0
(a-2)(a-3) < 0
(a -3)(3a+4) < 0
2