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.
sum(r/(12-r)-(r-1)/(13-r),r,1,6)
sum(r/(12-r)-(r-1)/(13-r),r,1,6)
Natural Language
Math Input
Extended Keyboard
Examples
Upload
Random
