A question. Finding the intercepts in -5x^2+20x+2... Having some issues.

So I've got -5x^2+20x+2, and have to determine the x intercepts.

I can use partial factoring to get -5x(x-4)+2

from which I can get

0=-5x

0/-5=x

0=x

AND

0=x-4

4=x

Which gives me 0 and 4.

My issues that when I enter -5x^2+20x+2 into a graphing calculator, the x-intercept coordinates I am given are -0.098 and 4.098. I'd like to know why. Have I done something wrong?

Thanks so much for the help!

Re: A question. Finding the intercepts in -5x^2+20x+2... Having some issues.

The x-interceopt are the values for x where -5x^2+20x+2 = 0. The mistake you made is going from -5x(x-4)+2=0 to -5x(x-4) = 0; what happened to the 2? You should always double check your answers to see if they work. If you plug x=0 or x = 4 into the original equation you get:

$\displaystyle -5(0^2)+20(0)+2 = 0$,

$\displaystyle 2 = 0$

and:

$\displaystyle -5(4^2)+20(4)+2 =0$,

$\displaystyle 2 = 0$

so clearly these answers are not right.

Easiest way to find the roots is to apply the quadratic equation:

$\displaystyle x = \frac {-20 \pm \sqrt {20^2-4(-5)(2)}} {2(-5)}$

Re: A question. Finding the intercepts in -5x^2+20x+2... Having some issues.

Quote:

Originally Posted by

**OverlyFrustrated** So I've got -5x^2+20x+2, and have to determine the x intercepts.

Well the roots are $\displaystyle 2\pm$$\displaystyle {\sqrt{110}/{5}$

Re: A question. Finding the intercepts in -5x^2+20x+2... Having some issues.

Hi there,

Thanks for taking the time to respond.

I've read the question I was basing this answer off of again and realize that there was no C value in the example I was provided.

In addition, it appears I was doing all kinds of bad math with the quadratic formula.

Thank you very much for your help.