Catenary

It has been long time since I posted last one.

I was thinking about catenary (Not only this one).
When I was a child, I thought the shape of a hanging rope was the parabola.
But, it is actually the other like called catenary.

Catenary is expressed like this.
y=a cosh(x/a)

The method how to derive the expression is explained here. (this page is Japanese)
http://aozoragakuen.sakura.ne.jp/taiwa/taiwaNch04/node48.html