The following appears in a text (in the context of generalizing Gauss' famous proof): enter image description here

I would like to reproduce this, but I cannot figure out how to get the alignment down. I thought of trying to use alignat but that does not seem to help (or maybe I simply do not know how to use it properly). Also, I would like for the horizontal line to be placed properly, but this is what I have come up with so far which is rather poor:


S_n &=& a &+& (a+d) &+\cdots+& [a+(n-1)d]\\
S_n &=& [a+(n-1)d] &+& [a+(n-2)d] &+\cdots+& a\\\hline
2S_n &=& [2a+(n-1)d]&+& [2a+(n-1)d &+\cdots+&[2a+(n-1)d]


which gives enter image description here

Any ideas on how to obtain the first image effectively?

1 Answer 1


You're better using an array (with some tricks):



\begin{array}{ r *{5}{ >{{}}c<{{}} c } }
 S_n &=& a           &+& (a+d)      &+& \cdots &+& [a+(n-1)d]\\
 S_n &=& [a+(n-1)d]  &+& [a+(n-2)d] &+& \cdots &+& a\\
2S_n &=& [2a+(n-1)d] &+& [2a+(n-1)d &+& \cdots &+& [2a+(n-1)d]


enter image description here

The >{{}}c<{{}} columns ensure correct spacing around the equals and plus signs, together with \arraycolsep set to zero.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .