The figure shows triangle ABC. AB and AC each carry one tick, and BC carries two ticks. What kind of triangle is ABC?
Two equal sides, so ABC is isosceles.
Ans: isosceles
Example 1
The figure shows triangle GHJ, with no marks on it. Nadia says that GHJ is an isosceles triangle because GJ and HJ look the same length. Is Nadia right?
No. There is not enough information to say GHJ is isosceles.
Ans: No
How to solve it
Read the marks in matching sets. Same mark, same length. A different mark means a different length.
In triangle ABC, AB and AC each carry one tick, so they are equal. BC carries two ticks, so it is different.
Two equal sides make an isosceles triangle. Three equal sides make an equilateral triangle, with every angle 60 degrees.
Angles name the others: a small square is a right angle, acute-angled means all angles under 90 degrees, obtuse-angled means one over 90.
If nothing is marked, there is no evidence. Triangle GHJ may look isosceles, but without matching ticks you cannot say so.
Two sides that look alike are still just two sides that look alike. A triangle's properties need evidence, not just appearance.
Practice questions with worked solutions
Question 1 is free to try. The other 9 open in the free trial, each with its full working.
88° is less than 90°, so angle ABC is an acute angle
47° and 45° are both less than 90°, so angle BCA and angle CAB are acute angles too
all 3 angles of ABC are acute angles
a triangle whose 3 angles are all acute is an acute-angled triangle
Answer: acute-angled
For parents
How do you name the triangle in P5 types of triangles questions?
This page teaches the properties of isosceles, equilateral and right-angled triangles from the Primary 5 Geometry strand, along with the names acute-angled and obtuse-angled triangle.
What your child learns here
This page teaches the properties of isosceles, equilateral and right-angled triangles from the Primary 5 Geometry strand, along with the names acute-angled and obtuse-angled triangle. Your child names a triangle only from what the figure states.
An isosceles triangle has two equal sides, and the angles opposite them, the base angles, are equal.
An equilateral triangle has three equal sides and three angles of 60°.
A right-angled triangle has one angle of 90°.
An acute-angled triangle has all three angles less than 90°; an obtuse-angled triangle has one angle greater than 90°.
The angles of any triangle add up to 180°.
Does the answer make sense?
The three angles of a triangle must add up to 180°, so a triangle can never have more than one right angle or one obtuse angle. A name your child gives must rest on what the figure marks or states, not on two sides that only look equal.
How to practise these questions
Basic questions train naming a triangle from its angles, right-angle square or tick marks rather than from how it looks. Intermediate questions add a right-angled isosceles triangle, read from two marks together. Challenge questions turn the figure on its side or ask your child to judge a claim or supply a missing mark. If a triangle is misnamed, watch that question's solution film and try it again from Revision later.
Questions parents ask
Can a triangle be both isosceles and right-angled? Yes. A right-angled isosceles triangle has a 90° angle and two equal sides, so its other two angles are 45° each.
Is an equilateral triangle also isosceles? An equilateral triangle has all three sides equal, so it does have two equal sides. The more exact name for it is equilateral.
Which triangles are in the Primary 5 syllabus? The Primary 5 Geometry strand lists the properties of isosceles, equilateral and right-angled triangles, the angle sum of a triangle, and finding unknown angles.
