Wolfram Research
Wolfram Math World: Inverse Tangent
This site is all about inverse tangents. The site includes a definition, color graphs, formulas, and many links to related terms.
Other
Lifewire: Excel Tan Function: How to Find the Tangent Angle
This article shows you how to find the tangent angle using Excel. You may be able to accomplish this on a scientific calculator, but if you don't have one handy, Excel can help you make these calculations better than anything else.