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All triangles have internal angles that add up to 180°, no matter the type of triangle.

• An isosceles triangle will have two angles the same size.
• In an equilateral triangle, all angles will be 60°.
• A right-angled triangle will have one angle that is 90°, which means the other two angles will have a total of 90°.
• A scalene triangle will have all angles of a different size.

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°

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°

Angles on a straight line also add up to 180°.

If a straight line has been split into three separate angles, it looks as if three corners of a triangle have been put together - just like in this video from 91�ȱ� Bitesize:

So if you had a missing angle on a straight line, you would apply the same logic as finding the missing angle in a triangle.

You know that one of the angles is 90°, represented by the right angle symbol, and another is 35°. And you also know that angles on a straight line add up to 180°.

To find the unknown angle, first add the known angles together:

90° + 35° = 125°

Now subtract from 180°:

180° - 125° = 55°

The missing angle above is 55°.

Use what you know about angles in triangles to find the missing angles in this quiz.

The triangles are not drawn to scale so you can't use a protractor.

Put your knowledge of angles in triangles to the test in this quiz.