Uh oh! Wolfram|Alpha doesn't run without JavaScript.
Please enable JavaScript. If you don't know how, you can find instructions
here
.
Once you've done that, refresh this page to start using Wolfram|Alpha.
the sum of 2*(L*N%2Bx) for x %3D 1 to (N-1)%2F2
Natural Language
Math Input
Extended Keyboard
Examples
Upload
Random