(n*(n-1))/2=105

Solution for (n*(n-1))/2=105 equation:

(n(n-1))/2=105

We move all terms to the left:

(n(n-1))/2-(105)=0

We multiply all the terms by the denominator


(n(n-1))-105*2=0


We calculate terms in parentheses: +(n(n-1)), so:

n(n-1)

We multiply parentheses

n^2-1n

Back to the equation:

+(n^2-1n)


We add all the numbers together, and all the variables

(n^2-1n)-210=0

We get rid of parentheses

n^2-1n-210=0

a = 1; b = -1; c = -210;
Δ = b2-4ac
Δ = -12-4·1·(-210)
Δ = 841
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$n_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$n_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{841}=29$
$n_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-1)-29}{2*1}=\frac{-28}{2}
=-14
$
$n_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-1)+29}{2*1}=\frac{30}{2}
=15
$