The same perimeter.
The same area?
A rectangle has a perimeter of 24 cm. Could its area be greater than 36 cm²?
Examples suggest an answer. How do we know it holds for every rectangle?
01 · Find a way into the problem
Ask: “What does the perimeter tell us about two adjacent sides?” They add to 12 cm. Calling one side x makes the other 12 − x.
That is the first useful move: translate the words into a relationship.
02 · Explain why the answer must hold
The area is x(12 − x) = 36 − (x − 6)².
A square cannot be negative, so the area cannot exceed 36 cm². Equality occurs at x = 6. We have moved from testing a few cases to explaining every case.
03 · Try a fresh question independently
Now change the perimeter to 28 cm. What is the greatest possible area? Can your child explain why, without following the previous working line by line?
Check: the sides add to 14; the area is 49 − (x − 7)², so the maximum is 49 cm².
What I’m looking for: choosing a representation, explaining a step and transferring the idea to a new question.