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.
\frac{4 + 1 + 3n}{2} (n+1) = 531
\frac{4 + 1 + 3n}{2} (n+1) = 531
Natural Language
Math Input
Extended Keyboard