Thread: Complete the square to find the vertex

Given: F(x)= x^2-6x-8 , complete the square to find the vertex.

TY for your help it is much appreciated!

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Originally Posted by duBxStep

Given: F(x)= x^2-6x-8 , complete the square to find the vertex.

TY for your help it is much appreciated!

Have you tried completing the square?

Well, I looked up how to do it on google and this is where I got:

y=a(x-h)^2+K
Y=1(x-h)^2+k
Y+8=x^2-6x
y+8-3=x^2-6x-3

but I have no idea where to go from there, and im not even sure if i'm on the right track.

Originally Posted by duBxStep

Given: F(x)= x^2-6x-8 , complete the square to find the vertex.

TY for your help it is much appreciated!

Have you been taught how to complete the square? What do you class notes and textbook have to say about it?

Think about this:

is the same thing.
is also the same thing.

So what you need to figure out is what number added to itself makes -6. That is: and if you square it what number do you get? .

What ever that ? is you need to make the -8 in your equation equal that by adding and subtracting the some number to it. and splitting it up like above.

Originally Posted by takatok

Think about this:

is the same thing.
is also the same thing.

So what you need to figure out is what number added to itself makes -6( ) and if you square it what number do you get? .

What ever that ? is you need to make the -8 in your equation equal that by adding and subtracting the some number to it. and splitting it up like above.

I thought that the formula was ? How come the A in the given formula has shifted over to the right one place?

Okay, so -6(x+x=-6) x=1/2 for -6 to = -6 right? square it and you get 1/4.

So I need to make 1/4 = to -8? Man im confused. I know I've done this before I just forgot how to do it!

So here I am now, let me know if I'm on the right track:

You've got it exactly. My original post was spaced badly. I corrected. Your method you just did is essentially the same as what I described.

So now from your earlier formula


a=1, h=3, and k=-17. Fill that in with whatever you were working on.

*Add the square of 3 to both sides; obtained by halving middle term coefficient -6x = -3





Hence, coordinates of vertex is (3, -17)