(100-2x)(80-2x)=8000

Solution for (100-2x)(80-2x)=8000 equation:

(100-2x)(80-2x)=8000

We move all terms to the left:

(100-2x)(80-2x)-(8000)=0

We add all the numbers together, and all the variables

(-2x+100)(-2x+80)-8000=0

We multiply parentheses ..

(+4x^2-160x-200x+8000)-8000=0

We get rid of parentheses

4x^2-160x-200x+8000-8000=0

We add all the numbers together, and all the variables

4x^2-360x=0

a = 4; b = -360; c = 0;
Δ = b2-4ac
Δ = -3602-4·4·0
Δ = 129600
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{129600}=360$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-360)-360}{2*4}=\frac{0}{8}
=0
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-360)+360}{2*4}=\frac{720}{8}
=90
$