geometry · Year 5 · Suggested learning sequence
Classifying triangles
Triangles can be sorted two ways, so read which one the question asks. By sides: equilateral has all three sides equal, isosceles has exactly two equal sides, and scalene has no equal sides. Matching tick marks show equal sides. By angles: acute-angled has every angle less than 90°, right-angled has one angle of exactly 90° (shown by a small square), and obtuse-angled has one angle more than 90°. Turning a triangle never changes its type. If only two angles are given, find the third first: the three angles always add to 180°.
Sample questions
Is this triangle equilateral, isosceles or scalene?
- Look at the tick marks: sides with the same marks are the same length.
- All three sides have the same mark, so all three sides are equal.
- It is an equilateral triangle.
Answer: equilateral
Is this triangle equilateral, isosceles or scalene?
- Look at the tick marks: sides with the same marks are the same length.
- Two sides have the same mark, so exactly two sides are equal.
- It is an isosceles triangle.
Answer: isosceles
Is this triangle equilateral, isosceles or scalene?
- Look at the tick marks: sides with the same marks are the same length.
- Each side has a different mark, so no two sides are equal.
- It is a scalene triangle.
Answer: scalene