# MathJax for integer summation proof

I have this

$$\begin{array}{r} sum &= 1 + 2 + \ldots + (n - 1) + n \\[1ex] + \; sum &= n + (n-1) + \ldots + 2 + 1 \\ \hline 2\,sum &= (n +1) + (n+1) + \ldots + (n+1) + (n+1) \end{array}$$

for displaying the Gauss trick for summing integers proof, but the alignment is not great. Any suggestions on how to improve this?

• You have not aligned the right-hand column. If you align it to the left then it will align to the equals sign. Use \begin{array}{rl}. If you also want to align the plus signs then you can use the array to do this manually, by putting each plus sign and each number in its own cell. Feb 11 '19 at 17:49
• (incidentally, I would have used the align environment for what you wrote, rather than array: \begin{align*}sum&=1+2+\ldots+(n-1)+n\\...\\ \hline 2sum&=...\end{align*}. I would use array to align the plus signs, but that may not be good LaTeX practice...) Feb 11 '19 at 17:54

$$\begin{array}{rrcccccccccc} &\text{sum}&=&1&+&2&+&\dots&+&(n-1)&+&n\\ +&\text{sum}&=&n&+&(n-1)&+&\dots&+&2&+&1\\ \hline &2\;\text{sum}&=&(n+1)&+&(n+1)&+&\dots&+&(n+1)&+&(n+1) \end{array}$$
$$\begin{array}{rrcccccccccc} &\text{sum}&=&1&+&2&+&\dots&+&(n-1)&+&n\\ +&\text{sum}&=&n&+&(n-1)&+&\dots&+&2&+&1\\ \hline &2\;\text{sum}&=&(n+1)&+&(n+1)&+&\dots&+&(n+1)&+&(n+1) \end{array}$$