Find b and c so that y=-18x^2+bx+c

Question

It has a vertex (-1,-2)

Answer 1

The vertex of a parabola lies at the x coordinate of -b/2a.

Since we know the x of the vertex and we know the value of a, we can find the value of b

-1 = - b

(2)(-18)

Solve for b.

-1 = - b

-36

36 = -b

b = -36

Now put both coordinates of the vertex into y = -18x² - 36 x + c and solve for c.

You can check by graphing.

Answer 2

y = a(x – h)² + k is the vertex form of a quadratic equation, where a is the leading coefficient

and h and k are the coordinates of the vertex point (h,k).

You're given a quadratic equation where a = –18, h = –1, and k = –2. So

y = –18(x + 1)² – 2 = –18(x² + 2x + 1) – 2

Distribute the –18 and collect like terms to put your quadratic equation in ax² + bx + c form, and you'll have your values of b and c.

Hope this helps.