Find the missing stretch
Use a known perimeter and known side lengths to find one missing side.
Explore this lesson →8 practice questions · Optional read aloud · Thinking pad
Let’s understand the idea.
- Perimeter is the distance around a polygon's entire boundary. Include each outside side once, including sides around an inward corner.
- Follow neighboring sides back to your starting point. Shared inside cut lines are not part of the outside boundary.
- When the whole perimeter and all but one side are known, the missing side is the remaining length.
- Area uses square units; perimeter uses length units. Rectangles can have the same area with different perimeters, or the same perimeter with different areas.
- Use the supplied side measurements. A drawing is not an actual-size ruler, and slanted lengths are given rather than requiring a new triangle theorem.
Something to notice
Observe physical boundary tracing, independent length measurement and a learner's explanation of why inside cuts are excluded. Structured tracing accepts any starting side and either continuous direction; mathematical answers use given exact side lengths. Rectangle comparisons evaluate actual dimensions and both measures, accepting all valid choices within the disclosed 1–12 side bounds. These bounded examples do not establish an unrestricted optimization theorem. Construction, independent explanations, retention and physical measurement require observation; a completed practice pack is not proof of mastery.
Try it. Change it. Explain it.
Open the lesson to use its pictures and controls. In How it works, connect the picture to the idea. In Play & discover, change the quantities and notice what stays the same. In Your turn, answer questions and use the explanation to check your reasoning.
You can ask for help, draw on the thinking pad, or listen to the question and answer options. Progress distinguishes independent work from work completed with help.
Let’s try it →Curriculum connection
Grade 3 · 3.MD · US Common Core
This lesson is mapped to these standards. A completed activity is practice evidence, not a claim of mastery. Content is checked against the linked official standards and its mathematical models are tested.