x(x+1)=1200

Solution for x(x+1)=1200 equation:

x(x+1)=1200

We move all terms to the left:

x(x+1)-(1200)=0

We multiply parentheses

x^2+x-1200=0

a = 1; b = 1; c = -1200;
Δ = b2-4ac
Δ = 12-4·1·(-1200)
Δ = 4801
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(1)-\sqrt{4801}}{2*1}=\frac{-1-\sqrt{4801}}{2}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(1)+\sqrt{4801}}{2*1}=\frac{-1+\sqrt{4801}}{2}
$