Math Is Fun Forum

Ridley-C

Member

Registered: 2015-07-31

Posts: 4

Polynomials?

The polynomial

has degree 3. If $f(-1) = 15$, $f(0)= 0$, $f(1) = -5$, and $f(2) = 12$, then what are the $x$-intercepts of the graph of $f$?
I have no idea what to do.

EDIT: i'm an idiot sorry XP

Last edited by Ridley-C (2015-09-02 07:23:03)

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Polynomials?

Hi;

You can put your math between the math tags. $ does not work here.

The x intercepts are -4, 0, 3/2

---

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

Bob

Administrator

Registered: 2010-06-20

Posts: 10,643

Re: Polynomials?

hi Ridley-C

There's no reason why you shouldn't just write it:

f(-1) = 15, f(0) = 0 etc.

So you need to solve these:

etc

d comes out straight away and it is not too hard to find the rest.

Once you have the equation you can factorise it to find the intercepts.

Bob

---

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

Gingerila

Member

Registered: 2015-08-24

Posts: 3

Re: Polynomials?

PLEASE HELP ME ANSWER THESE QUESTION!!!!!!

15. At Generic High School, in the year 2001, there were 20 students per every computer. In 2008, there were 4 students per computer. Write two ordered pairs to represent the ratio of student to computer. (Hint: Let x equal the number of years since 2000.)

16. Find the slope-intercept form of the equation that goes through these two points.

17. Find the point-slope form of the equation that goes through these two points.

18. During which year was there 10 students per computer?

Bob

Administrator

Registered: 2010-06-20

Posts: 10,643

Re: Polynomials?

hi Gingerila

Welcome to the forum.

Have you got the coordinates yet for Q15.

If yes, please post them.  If no, I'll go back to basics on graphs. 

Bob

---

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

Gingerila

Member

Registered: 2015-08-24

Posts: 3

Re: Polynomials?

Yeah, my coordinates are (1, 20) and (8, 4), I think.

Bob

Administrator

Registered: 2010-06-20

Posts: 10,643

Re: Polynomials?

hi Gingerila

Excellent!  Both correct.

The slope intercept equation is one like this:

m is the slope and c is the intercept.

You can read about it here:

http://www.mathsisfun.com/equation_of_line.html

In this case m will be negative as the line is sloping downwards and c will be more than 20 as it is the point where the line crosses the y axis.

You can also test yourself here:

http://www.mathsisfun.com/straight-line … ulate.html

I've just tried that and both m and c look like 'nasty' decimals.  Actually, its not as bad as it looks because those decimals are fairly straight forward fractions.

To do the next bit look here:

http://www.mathsisfun.com/algebra/line- … slope.html

And finally to get when y = 10, just look for the point on the line where y = 10, or solve this 10 = mx + c with your values for m and c.

Bob

---

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

Gingerila

Member

Registered: 2015-08-24

Posts: 3

Re: Polynomials?

thanks a million