SOLUTION: The vertex of a parabola is (-2, -20), and its y-intercept is (0, -12). The equation of the parabola is y = __x^2+__x+__

Algebra ->  Quadratic Equations and Parabolas -> SOLUTION: The vertex of a parabola is (-2, -20), and its y-intercept is (0, -12). The equation of the parabola is y = __x^2+__x+__       Log On


   



Question 1113237: The vertex of a parabola is (-2, -20), and its y-intercept is (0, -12).
The equation of the parabola is y = __x^2+__x+__

Answer by Edwin McCravy(20055) About Me  (Show Source):
You can put this solution on YOUR website!
The vertex of a parabola is (-2, -20), and its y-intercept is (0, -12).
We draw the approximate graph, using what is given:



The standard form of a quadratic parabola is

y=a%28x-h%5E%22%22%29%5E2%2Bk, where (h,k) is the vertex.

Since it is given that the vertex (h,k) = (-2, -20),

h = -2, and k = -20.

And since it passes through the point (0, -12),

x = 0 and y = -12

Substituting in

y=a%28x-h%5E%22%22%29%5E2%2Bk,

-12=a%280-%28-2%29%5E%22%22%29%5E2%2B%28-20%29

-12=a%280%2B2%5E%22%22%29%5E2-20

-12=a%282%5E%22%22%29%5E2-20

-12=a%284%29-20

-12=4a-20

8=4a

2=a

Now go back to the standard form:

y=a%28x-h%5E%22%22%29%5E2%2Bk

Substitute a = 2, h = -2, k = -20

However this time, we do not substitute anything
for x and y, for we want to leave them as variables:

y=a%28x-h%5E%22%22%29%5E2%2Bk

y=2%28x-%28-2%29%5E%22%22%29%5E2%2B%28-20%29

Then we simplify:

y=2%28x%2B2%5E%22%22%29%5E2-20

y=2%28x%2B2%5E%22%22%29%28x%2B2%5E%22%22%29-20

y=2%28x%5E2%2B4x%2B4%29-20

y=2x%5E2%2B8x%2B8-20

y=2x%5E2%2B8x-12

Edwin