How do you find the vertex and the intercepts for f(x)=x2−8x−9?

1 Answer
Nghi N.
Jul 19, 2016

Vertex (4, - 25)

Explanation:

f(x)=x2−8x−9
x-coordinate of vertex:
x=−b2a=82=4
y-coordinate of vertex:
y(4) = 16 - 32 - 9 = -16 - 9 = - 25
Vertex (4, -25)
Make x = 0 --> y-intercept = - 9
To find x-intercepts, make y = 0 and solve the quadratic equation:
y=x2−8x−9=0.
Since a - b + c = 0, use shortcut. The 2 x-intercepts (real roots) are:
-1 and (−ca=9)

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