Triangles come in many flavors — learn them all!
Polygons are closed 2D shapes with straight sides. Triangles are the simplest polygons. We classify triangles by their sides (equilateral, isosceles, scalene) and by their angles (acute, right, obtuse). Every triangle's interior angles always sum to 180°.
By Sides
By Angles
A triangle can be both isosceles AND right — these classifications are independent!
A triangle has angles 45° and 90°. Missing angle = 180° − 45° − 90° = 45°. Since two angles are equal, it's isosceles. And since one angle is 90°, it's a right triangle.
Remember This!
Tear the corners off a paper triangle and arrange them in a row — they always form a straight line (180°)! This is a great way to remember the angle sum.
A triangle has two angles measuring 60° each. What is the third angle, and what type of triangle is it?