(n+1)(n+2)=420

Solution for (n+1)(n+2)=420 equation:

(n+1)(n+2)=420

We move all terms to the left:

(n+1)(n+2)-(420)=0

We multiply parentheses ..

(+n^2+2n+n+2)-420=0

We get rid of parentheses

n^2+2n+n+2-420=0

We add all the numbers together, and all the variables

n^2+3n-418=0

a = 1; b = 3; c = -418;
Δ = b2-4ac
Δ = 32-4·1·(-418)
Δ = 1681
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$n_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$n_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{1681}=41$
$n_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(3)-41}{2*1}=\frac{-44}{2}
=-22
$
$n_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(3)+41}{2*1}=\frac{38}{2}
=19
$