Topics · Triangles · Primary Maths
Isosceles triangle
say eye-SOSS-uh-leez
A triangle with at least two sides equal. The two angles opposite those sides are equal too, which is what makes it useful in a question.
- 3 properties, all measured from the drawing
- part of Triangles
01 What is always true
∠DAB means the angle at A — the middle letter is always the corner, and the outer two say which sides form it.
Two of the sides are the same length.
BC = CA (93.5)
Two of the angles are equal.
∠CAB = ∠ABC (70°)
All 3 angles add up to 180°.
True of every 3-sided shape.
02 Why it works
The pairing is what earns the marks: equal sides sit opposite equal angles. Give a child one angle of an isosceles triangle and the other two follow, because two of them match and all three total 180°.
Watch which angle you are given. If it is the odd one at the top, the other two are equal and share what is left. If it is one of the matching pair, the other of the pair equals it and the top angle is what remains.
03 Where it sits
These are all isosceles triangles
See the whole family on Triangles.
04 Check you have it
Two questions on what is above. Answering shows the working straight away.
In isosceles triangle ABC above, CA = CB and ∠BCA = 40°. What is ∠ABC?
Which is always true of an isosceles triangle?
More practice
Full Triangles sets, marked as you go, at every level the topic is taught.