The idea in brief. Name the solid from what its faces are, then count in runs. A prism has the same face at both ends with rectangles between them; a pyramid has one base and every other face is a triangle meeting at a point on top. Once the solid is named, the edges come in equal runs you can see - round the top, round the bottom, and the ones joining them - so nothing has to be counted off the ink, which is just as well, because some of the edges are drawn dotted.
Lesson video, with the working
Solids: prisms, pyramids and their edges
4 min 22 s
The film teaches 2 questions in turn. Each question shows first; its working is written as the film explains it.
The solid below rests on a rectangular face. The face at the front and the face at the back are the same triangle, and the hidden edges are shown as dotted lines. How many edges does the solid have?
The face at the front and the face at the back are the same triangle, the same shape and the same size, so the solid is a prism.
edges round the triangular face at the front = 3
edges round the triangular face at the back = 3
edges joining a corner at the front to the corner behind it = 3
number of edges = 3 + 3 + 3 = 9
Ans: 9 edges
Example 1
One face of the solid below is a rectangle and the other four faces are triangles. The hidden edges are shown as dotted lines. How many edges does the solid have?
Every face except the bottom one is a triangle, and all four of them meet at the same point at the top, so the solid is a pyramid.
It is not a prism: a prism has a second face at its far end, the same shape and the same size as the first, and this solid has a point there.
edges round the rectangular face at the bottom = 4
edges running from the corners of that rectangle up to the point at the top = 4
number of edges = 4 + 4 = 8
Ans: 8 edges
How to solve it
Look at the faces. Matching triangles at both ends, the same shape and size, identify the triangular prism.
Count the edges round each triangular face: 3 at the front and 3 at the back.
Count the joining edges: 3. The total is 3 plus 3 plus 3, which is 9 edges.
For the rectangular pyramid, the triangular faces meet at one point.
Count 4 edges round the base and 4 edges up to the point. The total is 8 edges.
Include the dotted edges in their groups. They are hidden edges of the same solid.
Name the solid from its faces. Count its edges in groups, including the hidden edges.
Practice questions with worked solutions
Question 1 is free to try. The other 6 open in the free trial, each with its full working.
All 6 faces are rectangles, so the solid is a cuboid.
edges round the top face = 4
edges round the bottom face = 4
edges joining the top and bottom faces = 4
number of edges = 4 + 4 + 4 = 12
Answer: 12 edges
For parents
How can I help my child with naming solids from drawings?
A drawing shows a solid on a flat page, so your child has to think beyond the nearest visible face.
What your child learns here
A drawing shows a solid on a flat page, so your child has to think beyond the nearest visible face. The Primary 4 syllabus includes recognising representations of cubes, cuboids, cones, cylinders, prisms and pyramids. Looking at the arrangement of faces, edges and vertices helps your child explain a name, including when part of the object is hidden or the view has been turned.
Recognise the six named kinds of solid from their defining features, rather than naming the whole object after the shape of one face.
Distinguish a prism's matching parallel end faces from a pyramid's base and triangular side faces meeting at one vertex.
Identify a flat face, an edge where flat faces meet and a vertex where edges meet when describing cubes, cuboids, prisms and pyramids.
Account for hidden parts when counting features in a representation, and distinguish the curved surfaces of cones and cylinders from flat faces.
Does the answer make sense?
A cuboid has six faces, including faces hidden behind the visible ones. If your child counts only the three faces seen in a usual corner view, ask where the opposite face of each lies; the completed count must include both faces in each opposite pair.
How to practise these questions
The Basic questions name a cone and cylinder or count the edges and faces of familiar solids, including parts hidden from view. Intermediate work counts vertices of a prism shown standing on one of its end faces. The Challenge questions ask your child to classify a group of solids or infer a prism's face count from its edges. Before counting, have your child name or describe the solid so the hidden parts can be included systematically. A face, an edge and a vertex are different features, so check which one the answer requires. When reviewing a classification question, ask for the shared property of the group and the reason the remaining solid does not share it.
Questions parents ask
How can my child distinguish a prism from a pyramid? Look for the organisation of the faces. A prism has two matching, parallel end faces connected by side faces, while a pyramid has one base and triangular side faces that meet at a common vertex. A triangle visible on the page does not settle the name, because both a triangular prism and a pyramid can show triangular faces.
What do dotted edges in a solid drawing mean? They usually show edges hidden from the chosen viewpoint. Those edges still belong to the solid and count when the question asks for its total features. Your child should use the named object and the diagram's convention together. Imagining the object turned round can help account for a rear vertex without treating the drawing as a flat polygon.
Does Primary 4 ask children to recognise cones and cylinders too? Yes. The MOE list for identifying two-dimensional representations includes both, alongside cubes, cuboids, prisms and pyramids. A cylinder has two flat circular ends and a curved surface; a cone has one flat circular base and narrows to a vertex. The later drawing and net outcomes specify their own lists, so recognition and construction do not have identical scope.
