Learn with Amar
Teaching video
A teaching recording for this chapter has not been published yet. Start with the worked example below and explore the related animations where available.
Grade 3 chapters and video availability01 · Read and understand
What you will learn
- Use a perimeter total to recover an unknown side.
- Explain your method and check what the result means in the stated situation.
Before you start
Place value through hundreds, addition and subtraction, equal groups, and equal shares.
Keep paper nearby. Read the question once for the context, then again to identify what is known and what you need to find.
Start with a question
A triangle has perimeter 21 cm. Two sides measure 6 cm and 8 cm. Find the third side.
Why this math matters
Use total boundary measurements to check or complete a labeled diagram. The worked example shows how to connect the situation, its mathematical representation, and a check on the result.
Set up the model
A useful answer starts with clear assumptions:
- Use the units, grouping rules, and reference whole stated in the question.
- Treat the supplied counts and measurements as exact within the classroom model, unless an estimate or a random outcome is requested.
02 · Work through the example
Follow the reasoning, one step at a time.
Try to predict the next step before reading it. After each calculation, explain why the operation makes sense and how it helps answer the original question.
Work through it with Amar
See this example unfold.
The complete worked example, one idea at a time.
Find a missing boundary length
PausedQuestion: Start with the question. Paused.
Question
Start with the question
A triangle has perimeter 21 cm. Two sides measure 6 cm and 8 cm. Find the third side.
Before you calculate
Read what is known and what you need to find. Make a prediction before moving to the first calculation.
Starts paused. Play advances through the full text at a reading pace; pause whenever you need more time. Previous, Next, and the phase buttons let you set your own pace. Playback pauses when this walkthrough leaves the screen or you switch tabs.
Your device’s reduced-motion setting keeps each phase still. Manual controls remain available. The full written solution stays below.
Represent the situation
Known sides: 6 + 8 = 14 cm
Combine the boundary already accounted for.
Connect the parts
21 − 14
The missing side supplies the rest of the perimeter.
State and check the result
Third side = 7 cm
Check: 6 + 8 + 7 = 21 cm; these lengths can form a triangle.
The result
Third side = 7 cm
Check: 6 + 8 + 7 = 21 cm; these lengths can form a triangle.
Common mistakes to catch
- The perimeter includes every outside side.
- A computed missing side must still fit the shape's geometric conditions.
03 · Practice independently
Try it before revealing the answer.
Use paper or a calculator as needed. Write your units and reasoning, then open the hint or explanation to check your approach.
Practice 1
A quadrilateral has perimeter 30 m and known sides 7, 6, and 9 m. Find the fourth.
Show a hint
Subtract the sum of known sides.
Reveal answer and explanation
8 m
30 − (7 + 6 + 9) = 8.
Practice 2
A square's perimeter is 36 cm. Find one side.
Show a hint
All four sides are equal.
Reveal answer and explanation
9 cm
36 ÷ 4 = 9.
Take the idea with you
Use total boundary measurements to check or complete a labeled diagram.
04 · Reflect and continue
Can you explain it in your own words?
Before moving on, explain the main idea without looking at the worked example. Try both practice questions, check your reasoning, and name one mistake you now know how to avoid. Return to a step if you still need support.
Up next: Same perimeter, different area
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