Question

The line $x = 4$ is an axis of symmetry of the graph of $y = ax^2 + bx + c.$ Find $\frac{b}{a}$.

Answer 1

Answer: b/a = -8

Explanation:

When we have a quadratic equation:

y = a*x^2 + b*x + c.

The vertex of this function (and the axis of symetry) is located at:

x = -b/2a

Now, we know that the axis of symmetry is at x = 4.

then we have:

4 = -b/2a

And we want to find b/a.

Then we start with:

-b/2a = 4.

first we multiply by 2 in each side:

2*(-b/2a) = -b/a = 2*4 = 8.

now we multiply by -1 in each side:

-1*(-b/a) = b/a = -1*8.

Then we have: b/a = -8