The angles in a triangle add to 180° is in the Primary 5 Properties of Triangles chapter. The lesson film is the one Mathweave teaches this skill with in Primary 6, and the practice set below is chosen for Primary 5.
Lesson video, with the working
Angles in an Isosceles Triangle
2 min 17 s
The film teaches 2 questions in turn. Each question shows first; its working is written as the film explains it.
ABC is a triangle and AB and AC are the same length. Angle BAC is 76°. Find angle ABC.
Angle ABC is 52°.
Ans: 52°
Example 1
DEF is a triangle and DE and DF are the same length. Angle EDF is 96°. Find angle x.
Angle x is 42°.
Ans: 42°
How to solve it
Look for the equal sides. The marks say A B and A C are the same length.
So the two angles opposite those sides are the same size.
The three angles in a triangle add up to 180 degrees. The top angle is 76 degrees.
The other two share what is left: 180 minus 76, all divided by 2.
Angle A B C is 52 degrees. Write the property in the bracket.
Two equal sides, and two equal angles. The three angles in a triangle are 180 degrees together. 96 degrees of that is the top angle. So the two equal angles are each half of what is left.
Practice questions with worked solutions
Question 1 is free to try. The other 4 open in the free trial, each with its full working.
∠TUV = 180° − 90° − 38° = 52° (angle sum of triangle TUV)
Answer: 52°
For parents
How do you find the missing angle in P5 angles in a triangle questions?
This page covers the angle sum of a triangle and finding unknown angles, from the Primary 5 Geometry strand.
What your child learns here
This page covers the angle sum of a triangle and finding unknown angles, from the Primary 5 Geometry strand. Your child takes known angles off 180°, shares what is left between equal base angles, and steps outside the triangle along a straight line.
The three angles inside any triangle add up to 180°.
In a right-angled triangle, the two other angles share 90° between them.
Tick marks on two sides show an isosceles triangle, and the two angles opposite those sides are equal.
Each angle of an equilateral triangle is 60°.
When one side is extended, the angle outside and the inside angle next to it make 180° along the line.
Does the answer make sense?
In Question 1, 90° + 38° + 52° makes 180°, so the three angles of triangle TUV fill the angle sum exactly. Beside a right angle, both of the other angles must be acute, and 38° and 52° are.
How to practise these questions
Question 1, free to everyone, is triangle TUV with its right angle at T and angle TVU of 38°, and your child finds angle TUV. The Basic questions train the angle sum with a right angle, the equal base angles of an isosceles triangle, and an equilateral triangle. The Intermediate questions extend a side past the triangle, so a straight line gives the first angle. If your child is stuck, open its worked solution and check which angles are equal before subtracting.
Questions parents ask
Why do the angles of a triangle add up to 180°? Tear the three corners off a paper triangle and place them side by side. They fit together along a straight line, and a straight line is 180°.
How does my child know a triangle is isosceles? The question says so, or the figure shows matching tick marks on two sides. Sides that only look equal are not enough.
Are these questions in the Primary 5 syllabus? Yes. The Primary 5 Geometry strand lists the angle sum of a triangle and finding unknown angles in a triangle without drawing extra lines.
