PDA

View Full Version : Vertex of the parabola

Janie
05-30-2006, 02:30 PM
Find the vertex of the parabola

f(x)=3x squared +18x-3

tkhunny
05-30-2006, 03:34 PM
Find the vertex of the parabola

f(x)=3x squared +18x-3f(x) = 3(x<sup>2</sup> + 6x - 1)

Can you "Complete the Square" on the part inside the parentheses? That will do it.

Janie
05-30-2006, 03:59 PM
f(x)=3(xsquared+6x-1)
no because I don't know why you put 6x in there
this is very difficult

tkhunny
05-30-2006, 04:01 PM
I didn't put the '6x' in there.

Let's review the Distributive Property of Multiplication over Addition.

3(x<sup>2</sup> + 6x - 1) = 3x<sup>2</sup> + 18x - 3

Does that ring any bells? If we're stuck on this important, fundamental property, it will be very difficult to move on.

Mrspi
05-30-2006, 04:35 PM
Find the vertex of the parabola

f(x)=3x squared +18x-3

For a parabola whose equation is
f(x) = ax<sup>2</sup> + bx + c

the x-coordinate of the vertex is -b/2a. Once you have the x-coordinate, substitute it for x in the equation, and evaluate to find f(x)....which is the y-coordinate of the vertex.

f(x) = 3x<sup>2</sup> + 18x - 3

a = 3, and b = 18. The x-coordinate of the vertex, -b/2a, is -18/2(3), or -18/6, or -3. Now substitute -3 for x:

f(-3) = 3(-3)<sup>2</sup> + 18(-3) - 3
Do the arithmetic on the right-hand side, and you'll have the y-coordinate of the vertex.

I hope this helps you.

Janie
05-30-2006, 04:46 PM
f (-3)=24

do I then divide by -3

if so is it f=-8

skeeter
05-31-2006, 09:59 AM
f (-3)=24

do I then divide by -3

if so is it f=-8

I have no idea what you are doing.

first of all, f(-3) does not equal 24, f(-3) = 3(-3)<sup>2</sup> + 18(-3) - 3 = -30.

second ... are you unfamiliar with function notation?

you do not "divide" by -3 ... the function evaluated at x = -3 is f(-3) = y = -30

the vertex of the parabola is located at (-3,-30).