Primary 4 · Mathematics
P4 rectangles and squares
Most marks in this topic ride on one decision: which sides of the shape must be equal. Get that wrong and the working still looks tidy, it is just answering about a different shape. Here is what the slip looks like, and the one question that catches it.
- Level
- P4
- Usually taught
- Term 2
- Covers
- 1 concept
What changes at P4
In Primary 4, your child learns what makes a rectangle a rectangle: every corner is a right angle, and the sides are equal in opposite pairs. A square keeps all of that and adds one more rule, all four sides match. They also draw both shapes on squared paper.
The useful surprise: a square is a special rectangle, with one extra rule. Questions lean on the properties more than the drawing. Hide a side, give the perimeter, and see whether your child knows which sides must match.
The sentence that costs the mark
A rectangle has a perimeter of 30 cm. Its longer side is 9 cm. How long is its shorter side?
What the working usually does
one share of4 equal sides=7.5 cm
Splits 30 cm four equal ways. Tidy, precise looking, and true only for a square, where all four sides match.
What the question asked
one share ofthe other pair=6 cm
The two 9 cm sides take 18 cm. The shorter pair shares what is left, 12 cm, so each is 6 cm.
Both columns are one clean division of the same 30 cm, so nothing looks careless. The slip is in the setup: a rectangle's sides match in opposite pairs, not all four. Dividing by 4 answers a question about a square that nobody asked.
The same question, worked properly
Say which sides must match
9 cm pairs with the side opposite itThis is the step that fails. This is the property doing the work. Opposite sides of a rectangle are equal, so there is a second 9 cm side. Skip this step and dividing by 4 feels natural, but that is a square's rule, not a rectangle's.
Take out the known pair
30 − 9 − 9 = 12 cmThe two long sides claim 18 cm of the perimeter. What is left belongs entirely to the other pair.
The shorter pair shares it
12 ÷ 2 = 6 cmTwo equal short sides share 12 cm. A quick check closes it: 9 + 6 + 9 + 6 = 30. The division by 4 the wrong column used would only close for a square.
How can you tell it has clicked?
Before any arithmetic, ask your child to point at the sides that must be the same. A child who taps the two long sides, then the two short ones, has the property. A child who circles the whole shape, or says all of them, is about to divide by 4.
A second, quieter check: "is a square a rectangle?" The answer is yes, a special one. A child who can say why has the properties sorted, not memorised.
One thing to try tonight
Any rectangle in the house works: a phone, a book, the table. "Which sides are the same?" Point, no measuring needed. It takes thirty seconds and it is the entire property.
When perimeter homework appears, one follow-up question does the checking for you: "could you divide by 4 here? What shape would that make it?" If they can answer that, the trap above cannot catch them.
What your child will meet in this topic
- Rectangles and Squares
Most Singapore schools introduce this in Term 2, though schools do differ by a term either way. If your child’s school runs a different order, you can set it in the schedule on your dashboard.
What it leads to
Next term's Area and Perimeter leans on this directly: composite figures hide their side lengths, and the missing ones are found exactly the way the worked example found the 6 cm, by knowing which sides must match.
In P5 the family grows to the parallelogram, rhombus and trapezium, each with its own property set. A child who is used to asking "which sides must be equal?" walks into that chapter already holding the right question.
More in Primary 4
Other topics in the same year, so you can see what sits either side of this one.
Struggling with the working rather than the shape itself? How to help your child with problem sums
See how your child is doing on this topic
StudyLah marks each step separately, so you can see whether it was the setup or the arithmetic that cost the mark.