write y=-3x^2+12x-21 in vertex form
A)y=-3(x-2)^2-9
B)y=-3(x+2)^2-9
C)y=-3(x+2)^2+9
D)y=-3(x-2)^2+9

Question

write y=-3x^2+12x-21 in vertex form
A)y=-3(x-2)^2-9
B)y=-3(x+2)^2-9
C)y=-3(x+2)^2+9
D)y=-3(x-2)^2+9

anonymous · Answer

Do you know the basic vertex form? That is without the integers in the equation?

anonymous · Answer

no i do not, sorry

anonymous · Answer

f(x) = a(x-h)^2 + k this is vertex form.  But to complete it, you have to complete the square. Do you remember that?

anonymous · Answer

no

anonymous · Answer

Okay, so right now you have your equation in general form.  To complete the square you take the x integer and divide by 2.  After you divide by two, you square the result to recieve the "completed square"

anonymous · Answer

So take 12/2 = 6. 6^2 = 36

anonymous · Answer

ok

anonymous · Answer

One second, looking at my notes.  Its been a while since I have found the vertex myself!

anonymous · Answer

Okay now that u have that you have take subtract the 36 from the other side to keep things equal.  Let me solve this so I know what you get

anonymous · Answer

36 from 21?

anonymous · Answer

You must add the 21 to zero and subtract 36 from the zero.

So it will be -3x^2 +12x = 21 - 36

anonymous · Answer

When you combine those two to get -15.  Now you can pull the 3 out.  -3(x-2)^2 - 9

anonymous · Answer

where do you get -9 from?