# Thread: How did they get this answer?

1. ## How did they get this answer?

The equation is: 8x^2 - 3150x - 360000 = 0

the answer is: x= 486.285

i tried to solve this problem using the quadratic equation but I get a negative # in the "sqrt of b^2 - 4ac" and my calculator gives me an error. Can I not use the quadratic equation to solve for x? How do you get this answer?

thanks

2. ## Re: How did they get this answer?

The equation is: 8x^2 - 3150x - 360000 = 0

the answer is: x= 486.285
I agree that "their" positive solution is close ...

$b^2-4ac = (-3150)^2 - 4(8)(-360000) = 21442500$

$x = \dfrac{3150 \pm \sqrt{21442500}}{16}$

$x \approx 486.2877945$

also, $x \approx -92.53779451$

3. ## Re: How did they get this answer?

Originally Posted by esinc52
The equation is: 8x^2 - 3150x - 360000 = 0

the answer is: x= 486.285

i tried to solve this problem using the quadratic equation but I get a negative # in the "sqrt of b^2 - 4ac" and my calculator gives me an error. Can I not use the quadratic equation to solve for x? How do you get this answer?

thanks
The answer you have given is correct. (There is a second answer which is negative and so may not be valid in the context of the original problem.)

a=8, b=-3150, c=-360000

$\displaystyle b^2 - 4ac = (-3150)^2 - 4*8*(-360000)$ is positive.

I'd say you have made the very common mistake of calculating $\displaystyle b^2 = -3150^2$ rather than $\displaystyle (-3150)^2$.

4. ## Re: How did they get this answer?

$8x^2 - 3150x - 360000 = 0$

$a=8$
$b=-3150$
$c=-360000$

$r_{1,2} = \dfrac{3150 \pm \sqrt{(-3150)^2 -(4)(8)(-360000)}}{2(8)} = \dfrac{3150 \pm \sqrt{21442500}}{16} = (486.288, -92.5378)$

5. ## Re: How did they get this answer?

Thank you for your help guys. I see where my mistake was. "c" should be -360000, and not 36000. Also "b" should be -3150, and not 3150. I was thinking that the "b" and "c" were still the same whether it was "ax^2 + bx + c = 0" or "ax^2 - bx - c = 0"