See also: Triangle exterior angle definition

Theorem: An exterior angle of a triangle is equal to the sum of the opposite interior angles.

In the figure above, drag the orange dots on any vertex to reshape the triangle. The exterior angle at B is always equal to the opposite interior angles at A and C.

Although only one exterior angle is illustrated above, this theorem is true for any of the three exterior angles.

Because an exterior angle is equal to the sum of the opposite interior angles, it follows that it must be larger than either one of them. Stated more formally:

Theorem: An exterior angle of a triangle is always larger then either opposite interior angle.

Drag the vertices of the triangle around to convince yourself this is so.

- Triangle definition
- Hypotenuse
- Triangle interior angles
- Triangle exterior angles
- Triangle exterior angle theorem
- Pythagorean Theorem
- Proof of the Pythagorean Theorem
- Pythagorean triples
- Triangle circumcircle
- Triangle incircle
- Triangle medians
- Triangle altitudes
- Midsegment of a triangle
- Triangle inequality
- Side / angle relationship

- Perimeter of a triangle
- Area of a triangle
- Heron's formula
- Area of an equilateral triangle
- Area by the "side angle side" method
- Area of a triangle with fixed perimeter

- Right triangle
- Isosceles triangle
- Scalene triangle
- Equilateral triangle
- Equiangular triangle
- Obtuse triangle
- Acute triangle
- 3-4-5 triangle
- 30-60-90 triangle
- 45-45-90 triangle

- Incenter of a triangle
- Circumcenter of a triangle
- Centroid of a triangle
- Orthocenter of a triangle
- Euler line

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