Question

Find the x-intercepts of the parabola with vertex (-5,80) and y-intercept (0,-45). Write your answer in this form (x of 1 , y of 1), (x of 2, y of 2). If necessary round to the nearest hundereth

Answer 1

A parabola is a quadratic function, and a quadratic can be expressed in vertex form, which is:

y=a(x-h)^2+k, where (h,k) is the vertex (absolute maximum or minimum point of the quadratic)

In this case we are given that (h,k) is (-5,80) so we have so far:

y=a(x--5)^2+80

y=a(x+5)^2+80, we are also told that it passes through the point (0,-45) so:

-45=a(0+5)^2+80

-45=25a+80  subtract 80 from both sides

-125=25a  divide both sides by 25

-5=a, so now we know the complete vertex form is:

y=-5(x+5)^2+80

The x-intercepts occur when y=0 so:

0=-5(x+5)^2+80  add 5(x+5)^2 to both sides

5(x+5)^2=80  divide both sides by 5

(x+5)^2=16  take the square root of both sides

x+5=±√16  which is

x+5=±4  subtract 5 from both sides

x=-5±4 so the x-intercepts are:

x=-1 and -9