Thread: 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!

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:

,


and:

,


so clearly these answers are not right.

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

Originally Posted by OverlyFrustrated

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

Well the roots are

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.