A=(20-2x)(15-2x)

Solution for A=(20-2x)(15-2x) equation:

=(20-2A)(15-2A)

We move all terms to the left:

-((20-2A)(15-2A))=0

We add all the numbers together, and all the variables

-((-2A+20)(-2A+15))=0

We multiply parentheses ..

-((+4A^2-30A-40A+300))=0


We calculate terms in parentheses: -((+4A^2-30A-40A+300)), so:

(+4A^2-30A-40A+300)

We get rid of parentheses

4A^2-30A-40A+300

We add all the numbers together, and all the variables

4A^2-70A+300

Back to the equation:

-(4A^2-70A+300)


We get rid of parentheses

-4A^2+70A-300=0

a = -4; b = 70; c = -300;
Δ = b2-4ac
Δ = 702-4·(-4)·(-300)
Δ = 100
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$A_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$A_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{100}=10$
$A_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(70)-10}{2*-4}=\frac{-80}{-8}
=+10
$
$A_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(70)+10}{2*-4}=\frac{-60}{-8}
=7+1/2
$