a - b + c = 0.(1)
4a + 2b + c > 0.(2)
a < 0 .(3)
(2) - (1) 3a + 3b > 0 .a + b > 0 ,b > 0
a + c = b > 0
- a + b + c = (a - b + c) - 2a + 2b = 2( - a + b) > 0
(b² - 2ac) - 5a² = b² - 2a(b - a) - 5a² = (a + b)(b - 3a) > 0
以上四式都成立!