Plane Trigonometry

Area of Triangle

The area of any triangle is one-half the product of any of its two sides and their included angle (angle between the sides).

A = ½ ab sin C = ½ ac sin B = ½ bc sin A

Area of Triangle in Terms of the Sides (Heron's Formula, Hero of Alexandria)

Sum of Interior Angles

A + B + C = 180°

Sine Law:

In any triangle, the ratio of any side to the sine of its opposite angle is constant. This constant ratio is the diameter of the circumscribing circle.

Law of Tangents

A statement about the relationship between the tangents of two angles of a triangle and the lengths of the opposing sides.

Mollweide's Formula

Sometimes referred to in older texts as Mollweide's equations, named after Karl Mollweide, is a set of two relationships between sides and angles in a triangle.