Angles in triangles | Year 6 Mathematics PDF Download

Angles in triangles

All triangles have internal angles that sum up to 180°, regardless of the type of triangle.

• Isosceles Triangle: An isosceles triangle has two angles of the same size.
• Equilateral Triangle: In an equilateral triangle, all angles are 60°.
• Right-angled Triangle: This type of triangle has one 90° angle, making the other two angles add up to 90°.
• Scalene Triangle: A scalene triangle has all angles of different sizes.

Examples

Example 1: How would you find two missing angles on an isosceles triangle?

Use your knowledge about isosceles triangles - two angles are always the same size.
Find out what the total missing value is first:
180° - 50° = 130°
Angles d and e must now both equal 130°. Since we know these angles are both equal, you can find their value by halving 130°.
130° ÷ 2 = 65°
So d = 65° and e = 65°

Example 2: Look at this scalene triangle. How would you work out the value of a?

Since you know that all angles in a triangle add up to 180°, you have to add up the values of the angles that you do know and then subtract them from 180°:
40° + 60° = 100°
180° - 100° = 80°
Therefore:
a = 80°

Angles on a Straight Line

• Angles on a straight line add up to 180°.
• If a straight line is divided into multiple angles, treat it like angles of a triangle put together.
• To find a missing angle on a straight line, add the known angles together.
• Subtract the sum of known angles from 180° to find the missing angle.
• For instance, if you know two angles are 90° and 35°, the missing angle would be 55°.