A=(20-2x)(30-2x)

Solution for A=(20-2x)(30-2x) equation:

=(20-2A)(30-2A)

We move all terms to the left:

-((20-2A)(30-2A))=0

We add all the numbers together, and all the variables

-((-2A+20)(-2A+30))=0

We multiply parentheses ..

-((+4A^2-60A-40A+600))=0


We calculate terms in parentheses: -((+4A^2-60A-40A+600)), so:

(+4A^2-60A-40A+600)

We get rid of parentheses

4A^2-60A-40A+600

We add all the numbers together, and all the variables

4A^2-100A+600

Back to the equation:

-(4A^2-100A+600)


We get rid of parentheses

-4A^2+100A-600=0

a = -4; b = 100; c = -600;
Δ = b2-4ac
Δ = 1002-4·(-4)·(-600)
Δ = 400
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$A_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$A_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{400}=20$
$A_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(100)-20}{2*-4}=\frac{-120}{-8}
=+15
$
$A_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(100)+20}{2*-4}=\frac{-80}{-8}
=+10
$