Explore the properties of triangles — how their angles relate, and how sides determine triangle type.
The three interior angles of any triangle always add up to 180°. This is true no matter the triangle's shape or size.
A triangle has angles 50° and 60°. Find the third angle.
180° − 50° − 60° = 70°
A triangle has two equal angles of 45° each. Find the third angle.
180° − 45° − 45° = 90°
By sides: equilateral (all sides equal), isosceles (two sides equal), scalene (no sides equal). By angles: acute (all angles under 90°), right (one angle exactly 90°), obtuse (one angle over 90°).
A triangle has sides 5cm, 5cm, and 8cm. What type is it based on its sides?
Isosceles, since two sides are equal
A triangle has angles 90°, 40°, and 50°. What type is it based on its angles?
Right triangle, since one angle is exactly 90°
1. A triangle has angles 70° and 55°. Find the third angle.
2. A triangle has all three sides equal to 6cm. What type is it?
3. A triangle has angles 100°, 40°, and 40°. What type is it based on its angles?
4. A triangle has sides 3cm, 4cm, and 5cm, all different. What type is it based on its sides?
5. In a triangle, one angle is twice another, and the third angle is 30°. Find the other two angles.