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(1 - (7.85*pi*10^2-981)/(2*981))
arccos(1 - (7.85*pi*10^2-981)/(2*981))
Natural Language
Math Input
Extended Keyboard