Angles in Triangles 11 Plus Practice
19 October 2026
Understanding angles in triangles
Angles in triangles are a key part of the 11+ maths syllabus. Your child will need to know that the three angles inside any triangle always add up to 180 degrees. This simple fact is the starting point for most exam questions.
Finding a missing angle
To find a missing angle in a triangle, add the two angles you know, then subtract that total from 180. For example, if two angles are 55° and 70°, add them to get 125°. Then do 180 − 125 = 55°, so the missing angle is 55°.
Common triangle types
Children should also recognise special triangles. An equilateral triangle has all angles equal to 60°. An isosceles triangle has two equal angles. In a right-angled triangle, one angle is always 90°.
Key insight: Encourage your child to write down the angle fact before starting the calculation. This helps avoid common errors, like forgetting the total is always 180° in a triangle.
Practising at home
Use diagrams, worksheets or interactive tools to give your child regular practice with different triangle questions. Mix in questions with missing angles, and ask your child to explain their working out. This builds confidence and accuracy for the exam.
Spotting mistakes
Watch out for rushed arithmetic or confusion between triangle and straight line angle facts. Remind your child that angles in a straight line also add to 180°, but these are not inside a triangle.
Supporting next steps
Once your child is confident with triangle angles, try linking to other angle topics, like angles in quadrilaterals or on straight lines. This helps build a stronger understanding across the geometry section of the 11+.
