quad polynomials problem (1 Viewer)

[Kaatie]

graph y=8-2x-x^2
so far i have worked out by the quadratic formula that x=-4,2 but how do i find the vertex? then graph it? i feel really dumb

 

[kingkiller48]

hi Kaatie,

Solving the equation wouldn't find you the vertex...

What you have to do is you have to use x= -b/2a (axis of symmetry)

You realise that the axis of symmetry lies on the vertex... Then when you find x, put it into the equation.

Here's the solution:

y=8-2x-x^2
x= -b/2a
= -(-2)/2(-1)
= 2/-2
=-1

Sub in x=-1 to y,
y= 8-2(1) - 1^2)
= 8-2-1
= 5

So the vertex should be (-1, 5)

Sorry if i still get it wrong... I'm only in year 11..

Good luck with your HSC!

 

[Kaatie]

we're not supposed to use axis of symmetry but thanks anyway

 

[JasonNg1025]

You'd differentiate it I guess

y = 8 - 2x - x²

dy/dx = - 2 - 2x

This = 0 for stationary points.

-2 ( x + 1 ) = 0

hence x = -1.

Sub this into y equation,
y = 8 + 2 - 1
This is 9... I think kingkiller did a -2 instead of +2.

d²y / dx² = -2, and hence this stationary point must be a maximum.

Now, you find the solutions, as you have done.

So now you draw your axis. Plot the point where the maximum goes, and plot the x-intercepts. Then draw a maximum at the appropriate point and some curvy lines going out, making sure you hit the x-intercepts.

If you're unsure about any of these concepts (like second derivative) I'll -try- to explain

 

[M@ster P]

Jason, are you 12 years old for real??

so are you some kind of child prodigy

 

[Aerath]

OMFG. A 12 year old differentiating. *dies*

 

[kaz1]

we're not supposed to use axis of symmetry but thanks anyway

You can solve it and find the average between the x intercepts.

 

[tacogym27101990]

OMFG. A 12 year old differentiating. *dies*

that means he was like 11 last year doing MX2.
thats ridiculous

 

[lyounamu]

that means he was like 11 last year doing MX2.
thats ridiculous

That guy is getting trained under the idea of "no life" from yr 7.

 

[Graceofgod]

hmmm, a guy in yr 11 at my school finished 4u in year 9 and just finished a mathematics course at uni.

 

[tacogym27101990]

That guy is getting trained under the idea of "no life" from yr 7.

yeah i know right
go out on the weekend and shit ey
when i was 11 the only thing i knew about it was that it gave my sis nightmares haha