Let f(x)=ax2+bx+c. Consider the following diagram. Then
c < 0
b > 0
a + b - c > 0
abc < 0
Let f(x)=ax2+bx+c.
From the diagram, we can see that a > 0, c < 0 and -[b(2a)] < 0. Hence, b > 0.
∴ a+b−c>0