Question

Solve the equation for the indicated variable (Enter your answer as a common separated list)

S=n(n+1)/3 ; for n

Answer 1

You should remember that for a second grade equation,
\[ax^2+bx+c=0\]The solution is,
\[x=\frac{-b\pm\sqrt{b^2-4ac}}{2a}\]With this, you can now solve for n,
\[S=(n^2+n)/3\Rightarrow n^2+n-3S\]
Apply the solution with a=1, b=1, c=-3S.

Answer 2

The last equations should read this way,
\[n^2+n−3S=0\]