# Find all polynomials p(x)

• Dec 20th 2012, 01:51 PM
ohYeah
Find all polynomials p(x)
Hi so I was doing this question on my own for a while now, and I really can't find a way to answer part b)
I looked everywhere in the textbook nothing similar.
do you guys think the question is missing something, or am i missing my brain!

here it is
Attachment 26314

• Dec 20th 2012, 04:03 PM
Deveno
Re: Find all polynomials p(x)
first, we have to see how this relates to the matrix A.

what does the matrix A do to a polynomial p(x) = ax2+bx+c = [a,b,c] in the basis {x2,x,1}?

it takes [a,b,c] to [a-b+c,a+b+c,25a+5b+c].

note that p(-1) = a(-1)2 + b(-1) + c = a-b+c

p(1) = a+b+c

p(5) = a(52) + b(5) + c = 25a+5b+c.

so A takes p(x) to q(x) = p(-1)x2 + p(1)x + p(5).

suppose we are just given q(x), then to find p(x) we take A-1(q(x)).

we are told by the problem that q(x) = -x2+3x+31, which in our basis is [-1,3,31]. find A-1 of this vector.

(the fact that A is invertible means there is only ONE polynomial p(x) in P2 with this property).
• Dec 20th 2012, 06:52 PM
ohYeah
Re: Find all polynomials p(x)
what can i say Deveno your always there to help

thank you very much