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Finding polynomial coeffecients

If (x+2) is a factor of x^3+px^2-x-2 find p and the other factors when p has this value.

I used synthetic division and identified that 4p+2x-10=0; 4p+2x=10

HENCE

2p+x=5

Beyond that however, I am lost as to how I would go about finding the values of either P or X. My confusion stems from the fact that I have only been given one factor, namely, (x+2) and so it would appear that I am not able to utilise simultanteous equations to find the value of one term then substitute it into the equation to find the value of the other.
If (x-a) is a factor.

f(a) = 0
Original post by apronedsamurai
...


(x+2)(x2+kx1)=x3+px2x2(x+2)(x^2+kx-1)=x^3+px^2-x-2

Expand the brackets and compare coefficients of x2x^2 and xx.
Original post by apronedsamurai
If (x+2) is a factor of x^3+px^2-x-2 find p and the other factors when p has this value.

I used synthetic division and identified that 4p+2x-10=0; 4p+2x=10

HENCE

2p+x=5

Beyond that however, I am lost as to how I would go about finding the values of either P or X. My confusion stems from the fact that I have only been given one factor, namely, (x+2) and so it would appear that I am not able to utilise simultanteous equations to find the value of one term then substitute it into the equation to find the value of the other.



I don't think you're supposed to use simultaneous equations
I used synthetic division and directly found out that P=2
And using that value and I found the other factor to be x^2-1
Reply 4
Original post by binarythoughts
I don't think you're supposed to use simultaneous equations
I used synthetic division and directly found out that P=2
And using that value and I found the other factor to be x^2-1


How did you find that p=2?

When I used synthetic division, I used coeffecients 1 P -X and -2 with -2 as the input.

so, 1, p-2; -2+4-x 4p-10+2x

Even setting x to 0; in that case there is a discrepancy for P....:s-smilie:
Original post by apronedsamurai
How did you find that p=2?

When I used synthetic division, I used coeffecients 1 P -X and -2 with -2 as the input.

so, 1, p-2; -2+4-x 4p-10+2x


Could you first confirm if it's right or not from the mark scheme ?
Reply 6
p=2; (x+1)(x-1)

Is the answer given :smile:

So yes, your correct; which I had suspected from the beginning. When I gave my response, I was showing my working to identify where I had made a mistake :smile:
Original post by apronedsamurai
p=2; (x+1)(x-1)

Is the answer given :smile:

So yes, your correct; which I had suspected from the beginning. When I gave my response, I was showing my working to identify where I had made a mistake :smile:



Alright. The thing is I'm not sure how you arrived to those figures. You should directly get p=2 if you put in -2 as the input and your other inputs are correct.
You're final -8+4p=0
0 because they've stated it's a factor so you can just insert that and directly find P
Try and see if that helps
Reply 8
Original post by binarythoughts
Alright. The thing is I'm not sure how you arrived to those figures. You should directly get p=2 if you put in -2 as the input and your other inputs are correct.
You're final -8+4p=0
0 because they've stated it's a factor so you can just insert that and directly find P
Try and see if that helps


Did you include X as a coeffecient?
Original post by apronedsamurai
Did you include X as a coeffecient?


Oh, I just figured out your mistake!
You mentioned one of your values as -x
that is incorrect as you are supposed to mention only the coefficients not x. So the coefficient is -1
Original post by binarythoughts
Oh, I just figured out your mistake!
You mentioned one of your values as -x
that is incorrect as you are supposed to mention only the coefficients not x. So the coefficient is -1


Thank you! :biggrin:. Can't believe I did that :|
Original post by apronedsamurai
Thank you! :biggrin:. Can't believe I did that :|


Glad I could help

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