On the site calculator: http://web2.0calc.com/
You could either click "2nd" (top left corner) and then "atan" (fourth row, first column), or type in "atan(x)".
On the site calculator: http://web2.0calc.com/
You could either click "2nd" (top left corner) and then "atan" (fourth row, first column), or type in "atan(x)".