# Need help solving for:

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• Jan 2nd 2010, 11:38 PM
Vuong
Need help solving for:
the axis of symmetry, the vertex, any x-and y-intercepts and graphing the parabolas.

here's an example question

y=-4x^2-9x-2

also y=x^2-6x+25
• Jan 2nd 2010, 11:42 PM
Drexel28
Quote:

Originally Posted by Vuong
the axis of symmetry, the vertex, any x-and y-intercepts and graphing the parabolas.

here's an example question

y=-4x^2-9x-2

Let $f(x)=ax^2+bx+c$. A little completing the square shows that $f(x)=a\left(a+\tfrac{b}{2a}\right)^2+c-\frac{b^2}{4a}$. From this we see that the vertex is at $\left(\tfrac{-b}{2a},c-\tfrac{b^2}{4a}\right)$, the axis of symmetry at $x=\frac{-b}{2a}$, the x-intercepts are at $x=\frac{-b\pm\sqrt{b^2-4ac}}{2a}$ given $\Delta\geqslant0$, and the y intercepts are $f(0)=c$.
• Jan 2nd 2010, 11:47 PM
Vuong
I never understood what the f(x) meant...
• Jan 3rd 2010, 12:26 AM
11rdc11
Quote:

Originally Posted by Vuong
I never understood what the f(x) meant...

Same thing as y
• Jan 3rd 2010, 01:49 AM
Wilmer
Quote:

Originally Posted by Vuong
y=-4x^2-9x-2

Start by factoring -4x^2 - 9x - 2
• Jan 3rd 2010, 01:16 PM
Vuong
Quote:

Originally Posted by Wilmer
Start by factoring -4x^2 - 9x - 2

What do I do after?
• Jan 3rd 2010, 02:01 PM
Quacky
y=-4x^2-9x-2

Ok, the line will cut the y axis where x=0.
Substitute x=0 into the equation, and you get y=-2. So point of interception is at:
(0,-2)
Now, the line will cut the x axis where y=0. To find this, we can try to factorise:

$-4x^2-9x-2=0$
$4x^2+9x+2=0$
$(4x+1)(x+2)=0$
x=-2 and x=-1/4
x intercepts are
$(-2,0)$ and $(-1/4,0)$

Parabolas are symmetrical, so if you take the x intercepts, the line of symmetry will be halfway between them.
$-2.25/2 = -1.125$ (wow, nasty numbers)
x=-1.125 is the line of symmetry.

A question about the vertex: Have you covered differentiation?
• Jan 3rd 2010, 03:25 PM
Vuong
Quote:

Originally Posted by Quacky
y=-4x^2-9x-2

Ok, the line will cut the y axis where x=0.
Substitute x=0 into the equation, and you get y=-2. So point of interception is at:
(0,-2)
Now, the line will cut the x axis where y=0. To find this, we can try to factorise:

-4x^2-9x-2=0
4x^2+9x+2=0
(4x+1)(x+2)=0
x=-2 and x=-1/4
x intercepts are
$(-2,0)$ and $(-1/4,0)$

Parabolas are symmetrical, so if you take the x intercepts, the line of symmetry will be halfway between them.
$-2.25/2 = -1.125$ (wow, nasty numbers)
x=-1.125 is the line of symmetry.

A question about the vertex: Have you covered differentiation?

Why are they minus?
• Jan 3rd 2010, 03:30 PM
Quacky
(4x+1)(x+2)=0
Substitute x=-2 in and see what happens.
• Jan 3rd 2010, 03:33 PM
Vuong
Quote:

Originally Posted by Quacky
(4x+1)(x+2)=0
Substitute x=-2 in and see what happens.

Uhh (-7)(0) = 0?
• Jan 3rd 2010, 03:59 PM
Quacky
Yes.
If you substitute 2 into the equation, you get
$(9)(4) = 36.$
As you want the solutions to
$(4x+1)(x+2)=0$ (not 36)
x=-2 is the correct solution, not x=2.
• Jan 3rd 2010, 04:00 PM
Vuong
Quote:

Originally Posted by Quacky
Yes.
If you substitute 2 into the equation, you get
$(9)(4) = 36.$
As you want the solutions to
$(4x+1)(x+2)=0$ (not 36)
x=-2 is the correct solution, not x=2.

Great, thanks! Big help. Do you know how to factor a problem like 6x^2-5x-6? I've been trying a bunch of combinations, still unsuccessful
• Jan 3rd 2010, 04:05 PM
Quacky
Think of it this way:
http://www.mathhelpforum.com/math-he...505670c8-1.gif
For that to be true, either
$(4x+1)=0$ or $(x+2)=0$
If $4x+1=0$
$4x=-1$
$x=-1/4$

If $x+2=0$
$x=-2$

Edit: I'll look at the above problem now.
• Jan 3rd 2010, 04:05 PM
Drexel28
Quote:

Originally Posted by Vuong
Great, thanks! Big help. Do you know how to factor a problem like 6x^2-5x-6? I've been trying a bunch of combinations, still unsuccessful

$6x^2-5x-6=0\implies x=\frac{5\pm\sqrt{25-4(6)(-6)}}{12}={{{\tfrac{18}{12}}\brace{\tfrac{-8}{12}}}}$ so that $6x^2-5x-6=\left(x-\tfrac{18}{12}\right)\cdot\left(x+\tfrac{8}{12}\ri ght)$
• Jan 3rd 2010, 04:10 PM
Vuong
Quote:

Originally Posted by Drexel28
$6x^2-5x-6=0\implies x=\frac{5\pm\sqrt{25-4(6)(-6)}}{12}={{{\tfrac{18}{12}}\brace{\tfrac{-8}{12}}}}$ so that $6x^2-5x-6=\left(x-\tfrac{18}{12}\right)\cdot\left(x+\tfrac{8}{12}\ri ght)$

You could use quadratic to factor?
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