Find all the zeros of the function algebraically.

• May 1st 2013, 09:37 AM
captaincrunch383
Find all the zeros of the function algebraically.
Find all real zeros of the function algebraically.

f(x)= x^4 - x^3 - 15x^2

help anyone?
• May 1st 2013, 10:20 AM
Shakarri
Re: Find all the zeros of the function algebraically.
$\displaystyle x^4-x^3-15x^2=x^2(x^2-x-15)$
Can you find all the zeros in that?
• May 1st 2013, 10:35 AM
captaincrunch383
Re: Find all the zeros of the function algebraically.
Quote:

Originally Posted by Shakarri
$\displaystyle x^4-x^3-15x^2=x^2(x^2-x-15)$
Can you find all the zeros in that?

no i can not =/

can you help me, i'm getting desperate.
• May 1st 2013, 10:40 AM
sciencepal
Re: Find all the zeros of the function algebraically.
0,0,$\displaystyle 1\pm\sqrt{61}/2$
• May 1st 2013, 10:41 AM
captaincrunch383
Re: Find all the zeros of the function algebraically.
Quote:

Originally Posted by sciencepal
0,0,$\displaystyle 1\pm\sqrt{61}/2$

can you please show how you got that? this is just a sample problem and i'm goin to have a similar problem on my test, i'd appreciate it.
• May 1st 2013, 10:42 AM
dokrbb
Re: Find all the zeros of the function algebraically.
Shakarri meant that one, $\displaystyle x^2(x^2-x-15)$

now you have in the brackets a second degree equation, it's impossible that you can not find the zeros from that one, try it,

ps: oops, we might have answered in the same time
• May 1st 2013, 10:42 AM
captaincrunch383
Re: Find all the zeros of the function algebraically.
Quote:

Originally Posted by dokrbb
Shakarri meant that one, $\displaystyle x^2(x^2-x-15)$

now you have in the brackets a second degree equation, it's impossible that you can not find the zeros from that one, try it,

dokrbb

so you factored out an x squared, i see that, but i still don't know how to find the zeros from that one. =/
• May 1st 2013, 10:52 AM
Shakarri
Re: Find all the zeros of the function algebraically.
$\displaystyle x^2(x^2-x-15)$ is zero when either $\displaystyle x^2=0$ or $\displaystyle x^2-x-15=0$
Find the values of x which make either of those zero.
• May 1st 2013, 10:58 AM
captaincrunch383
Re: Find all the zeros of the function algebraically.
Quote:

Originally Posted by Shakarri
$\displaystyle x^2(x^2-x-15)$ is zero when either $\displaystyle x^2=0$ or $\displaystyle x^2-x-15=0$
Find the values of x which make either of those zero.

that sounds completely foreign to me =/

there is a similar problem to this on my test, and i'm trying to figure this out. i was hoping you or someone would briefly post the steps used to solve it. sorry if i sound rude
• May 1st 2013, 11:23 AM
captaincrunch383
Re: Find all the zeros of the function algebraically.
Never mind i got it.