CK-12 Foundation
Ck 12: Geometry: Inverse Trigonometric Ratios
[Free Registration/Login may be required to access all resource tools.] Use the inverse trigonometric ratios.
Wolfram Research
Wolfram Math World: Cut the Knot! Angles
Very good site with links to reviews of various angles.
Math Planet
Math Planet: Geometry: Mean and Geometry
The geometric mean is explained through examples and a video lesson. [1:12]