Lesson 2: Triangles

Property

Triangles can be classified by the specific sizes of their interior angles into three categories:

• Right Triangle: Contains exactly one right angle (90°).

• Obtuse Triangle: Contains exactly one obtuse angle (greater than 90°).

Property

• The sum of the angles in any triangle is $180\degree$.
• The base angles of an isosceles triangle are equal.
• All the angles of an equilateral triangle are equal.

Examples

• If a triangle has two angles measuring $40\degree$ and $80\degree$, the third angle must be $180\degree - (40\degree + 80\degree) = 60\degree$.
• An isosceles triangle has a vertex angle of $50\degree$. The two base angles are equal, so each one measures $(180\degree - 50\degree) \div 2 = 65\degree$.
• An equilateral triangle has three equal angles, so each angle must be $180\degree \div 3 = 60\degree$.

Explanation

If you tear off the three corners of any paper triangle and line them up, they always form a perfectly straight line, which measures $180\degree$. This universal rule helps you find any missing angle in a triangle.