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.
arccos((11^2+13^2-8.5^2)/(2*11*13))
arccos((11^2+13^2-8.5^2)/(2*11*13))
Natural Language
Math Input
Extended Keyboard
