Understand the idea.
Straight or curved?
Follow an edge with your finger. A straight edge keeps its direction; a curved boundary bends. A semicircle has both kinds: one straight edge and one curved part. Describe the particular part you mean when a shape has both.
Across from each other
Opposite sides lie across a shape and do not share a corner. Parallel sides lie along straight lines that keep the same gap if continued. A rectangle has parallel opposite sides, but other four-sided shapes may have opposite sides that slope towards each other.
See the method in action.
Find an opposite side
In rectangle ABCD, which side is opposite AB?
Worked solution
- 1
Locate side AB.
A is at the top left and B is at the top right, so AB is the top side.
- 2
Look across to the side that shares neither endpoint.
The bottom side joins C and D; it does not meet AB at a corner.
- 3
Name the opposite side.
CD is across the rectangle from AB, making it the opposite side.
CD.
Notice a parallel pair
Which labelled edges of the trapezium are parallel?
Worked solution
- 1
Look at the directions of the labelled edges.
The top and bottom edges both run straight across the page.
- 2
Check that their lines keep the same gap.
Continuing these horizontal lines would not make them meet, so they are parallel.
- 3
Choose the top and bottom pair.
The sloping side lines move closer together in one direction, so they are not another parallel pair.
The top and bottom edges.
Try it yourself.
Check your understanding
In a rectangle, does AB share a corner with its opposite side CD?
Show answer & reasoning
No.
AB and CD lie across the rectangle. They have no shared endpoint, so they do not share a corner.
Keep practising
6 worksheet questions
Read the questions online
- Is the boundary of a circle straight or curved?
- A semicircle is made by cutting a circle straight across its centre. Name the two kinds of boundary it has.
- In rectangle ABCD, name the side opposite AD.
- Name a side that shares a corner with AB.
- Name the parallel pair in the trapezium.
- Sam draws two sloping lines that get closer together. He says they are parallel because they have not met yet. Is he right? Explain.
Read the worked solutions
- Is the boundary of a circle straight or curved?
It bends all the way around.
Curved. - A semicircle is made by cutting a circle straight across its centre. Name the two kinds of boundary it has.
The cut is straight. The remaining half of the circle is curved.
One straight edge and one curved part. - In rectangle ABCD, name the side opposite AD.
AD is on the left. BC is across from it on the right.
BC. - Name a side that shares a corner with AB.
AD shares corner A; BC shares corner B. Either answer is correct.
AD or BC. - Name the parallel pair in the trapezium.
The horizontal edges keep the same separation.
The top and bottom edges. - Sam draws two sloping lines that get closer together. He says they are parallel because they have not met yet. Is he right? Explain.
Their gap changes, so they are not parallel even before they meet.
No.
Curriculum reference & lesson scope
Australian Curriculum v9 and SA Curriculum Mathematics
Sides and spatial features; straight/curved/opposite/parallel; quarter/half/three-quarter/full turns.
Official curriculum source ↗