x(x+1)/2=120

Solution for x(x+1)/2=120 equation:

x(x+1)/2=120

We move all terms to the left:

x(x+1)/2-(120)=0

We multiply all the terms by the denominator


x(x+1)-120*2=0

We add all the numbers together, and all the variables

x(x+1)-240=0

We multiply parentheses

x^2+x-240=0

a = 1; b = 1; c = -240;
Δ = b2-4ac
Δ = 12-4·1·(-240)
Δ = 961
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{961}=31$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(1)-31}{2*1}=\frac{-32}{2}
=-16
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(1)+31}{2*1}=\frac{30}{2}
=15
$