Find a Missing Side from a Perimeter

Supplied materials

• Paper and a pencil; drawing and spoken explanations are welcome. An adult may read directions and options aloud.
• Supplied worked-case record: A four-sided shape has perimeter 28 cm and three known sides 7,5,9 cm.
• All quantities and shape descriptions are supplied in the worked example and questions; no purchase or personal information is needed.

Learn the idea

A supplied polygon perimeter is the sum of all its boundary sides. If one side is unknown, add the known side lengths and subtract that subtotal from the whole perimeter. The unknown fills the missing boundary length. It is not a missing area or diagonal.

List the known sides and add them once. Subtract their subtotal from the supplied perimeter. Check by adding all sides, including your result, to reconstruct the full boundary length.

Include each known side once in the subtotal. Do not assume opposite sides match unless a shape definition supplies that fact. Draw a generic polygon and label the unknown side with a question mark.

1. Draw a generic polygon and label the unknown side with a question mark.
2. Follow the worked example. Point to each number or shape feature and say what it tells you.
3. List the known sides and add them once.
4. Try the question and then the three practice questions. Check your answers and explain any change you make.

Check your reasoning

Your answer is temporary and is not saved to an account. Download your current answer before leaving or reloading. Written work on paper or in another app stays in your own file.

Take it further

Perimeter 35 cm with other sides 9,8,7 cm gives missing side 11 cm.

Nothing here is saved to an account or sent to a teacher. The answer download includes your current choice and any checked feedback. It does not include choices from the separate More practice section. Download that section separately.