# Math Help - quadratic expression

The diagram shows part of the graph of the function: y=ax^2+bx+c.

(seeing as I cannot draw the diagram here, it is basically an upside down parabola, with the vertex on the x-axis, and the whole parabola is in the fourth quadrant)

State whether each of the following expressions is positive, negative or zero.

a) a negative
b) c negative
c) b^2-4ac zero
d) b ?

I cannot figure out how to find whether constant b is positive, negative or zero. Can someone help me?

2. I guess one way to do it is:

$y=a\left (x^2-\frac{b}{a}x\right )+c$

$y=a\left ( x-\frac{b}{2a}\right )^2 -\frac{b^2}{4a}+c$

So the graph would be translated $\frac{b}{2a}$ units to the right. But in your graph the vertex is to the left, and $a$ is negative. Therefore $b$ must be positive.