Seems legitimate to me

Line 1: $1 + 2 + 3 + ... + n = \frac{n(n+1)}{2}$

Line 2: $\displaystyle \sum_{n=1}^{\infty} n = \frac{-1}{12}$ (Considering Riemann zeta regularisation)

Line 3: (When $n = \infty$ ): $\frac{-1}{12} = \frac{\infty(\infty+1)}{2}$

Line 4: $\infty^2 + \infty + \frac{1}{6} = 0$

Line 5: $\infty = \frac{-(1) \pm \sqrt{(1)^2 - 4(1)(\frac{1}{6})}}{2(1)}$

Line 6: $\infty = \frac{-3 \pm \sqrt{3}}{6}$

Where does this proof first go wrong?