# Fining the Zeros of this polynomial.

• Apr 1st 2007, 09:39 PM
Mathfailure
Fining the Zeros of this polynomial.
Hello. I am having a hard time finding the Zeros to this polynomial function. Could you please explain the steps. I keep coming up with really odd answers when I factor that couldn't possibly be the zeros to this function. Thanks.

P(X)=-x^3+21x-20
• Apr 1st 2007, 09:51 PM
Jhevon
Quote:

Originally Posted by Mathfailure
Hello. I am having a hard time finding the Zeros to this polynomial function. Could you please explain the steps. I keep coming up with really odd answers when I factor that couldn't possibly be the zeros to this function. Thanks.

P(X)=-x^3+21x-20

Here
• Apr 1st 2007, 10:00 PM
Jhevon
Here is the graph, if you're interested
• Apr 1st 2007, 10:15 PM
Mathfailure
Do you have to divide? We haven't learned how to divide anything into a polynomial yet. Thanks for the quick reply
• Apr 1st 2007, 10:19 PM
Jhevon
Quote:

Originally Posted by Mathfailure
Do you have to divide? We haven't learned how to divide anything into a polynomial yet. Thanks for the quick reply

No you don't, but dividing makes life simplier. you can actually find the other roots the same way we found the first one. that is, we plug in the factors of the lone constant into the equation to see if we get zero.

the factors of 20 are: +/- 1, +/- 2,+/- 4,+/- 5,+/- 10, +/- 20

so i started by plugging in +1, and as luck would have it, that was a root, that is, P(1)=0. i then used that to divide. however, i could have continued by plugging in the other numbers to see which gives zero. eventually you would have found that +4 and -5 are roots