The <em><u>correct answer</u></em> is:
3 inches.
Explanation:
We will use the Rational Roots Theorem to solve this.
First we can divide both sides of the equation by 4 in order to simplify it:
![\frac{4x^3-72x^2+320x}{4}=\frac{420}[4} \\ \\x^3-18x^2+80=105](https://tex.z-dn.net/?f=%20%5Cfrac%7B4x%5E3-72x%5E2%2B320x%7D%7B4%7D%3D%5Cfrac%7B420%7D%5B4%7D%20%5C%5C%20%5C%5Cx%5E3-18x%5E2%2B80%3D105%20)
In order to solve this, we want the polynomial set equal to 0. To do this, subtract 105 from both sides:
![x^3-18x^2+80x-105=105-105 \\x^3-18x^2+80x-105=0](https://tex.z-dn.net/?f=%20x%5E3-18x%5E2%2B80x-105%3D105-105%20%5C%5Cx%5E3-18x%5E2%2B80x-105%3D0%20)
The Rational Roots Theorem says that if p/q is a root of the polynomial, then p is a factor of the constant term and q is a factor of the leading coefficient. The constant term is -105. Drawing a factor tree, we find that the factors of this number are 1, -1, 3, -3, 5, -5, 7, -5, 15, -15, 21, -21. The leading coefficient is 1; its only factor is 1. This means p/q must be a whole number, and can be any of the factors of 105.
Using synthetic division, we try 1 in the box:
<u>1 |</u> 1 -18 80 -105
_______<u> 1</u>__<u> -17</u>___<u>63</u>__
1 -17 63 -42
Since there is a remainder, this is not a root. Trying -1,
<u>-1 |</u> 1 -18 80 -105
_______<u>-1</u>__<u> 19 </u>_<u>-99</u>_
1 -19 99 -204
This is not a root; in fact, it shows us that none of the negatives will be a factor, as the absolute values increase as we complete the synthetic division.
Trying 3,
3 | 1 -18 80 -105
________<u>3</u>__<u>-45</u>___<u>105</u>_
1 -15 35 0
Since there is no remainder, 3 is a root, and is the answer we are looking for.