Plane Geometry
A few examples of what you can ask Wolfram|Alpha about:
Plane Figures
compute properties of a plane figure
specify parameters for a figure
compute the complement of an angle
compute the supplement of an angle
do computations with complementary angles
compute a reference angle
compute properties of a triangle with given side lengths
compute properties of a regular polygon
specify parameters for a regular polygon
get information about a planar lamina
find a moment of inertia for a lamina
Golden Ratio Formula
determine golden ratio numbers and lengths