The Triangle Angle Sum Property
The Triangle Angle Sum Property says that the sum of its internal angles is 180°. If A, B and C are the internal angles of the triangle, then: A+B+C = 180°.
How it can be used to determine the measure of angles?
For example, if you have a triangle with internal angles A, B and C (A=50° and B=40°), but you don't know the value of the angle C, you can calculate it using The Triangle Angle Sum Property, as follow:
A+B+C=180°
C=180°- A - B
C=180°- 50°- 40°
C= 90°