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.
arc cos ((12^2 + 15^2 - 18^2)/(2 * 12 * 15))
arc cos ((12^2 + 15^2 - 18^2)/(2 * 12 * 15))
Natural Language
Math Input
Extended Keyboard
