What is Perimeter?

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Grade Level:

Pre-School – Class 2

All domains without exception

Definition

What is it?

Perimeter is the total distance around the outside edge of a 2D shape. Imagine you're walking along the boundary of a park; the total distance you walk is its perimeter. It's simply the sum of the lengths of all the sides of a shape.

Simple Example

Quick Example

Think about your school's cricket ground. If you walk all the way around the boundary rope once, the total distance you covered is the perimeter of the cricket ground. If one side is 100 meters, another is 80 meters, and so on, you add all those lengths together.

Worked Example

Step-by-Step

Let's find the perimeter of a rectangular photo frame. The frame has a length of 20 cm and a width of 15 cm.
---Step 1: Identify the shape and its sides. It's a rectangle, so it has two lengths and two widths.
---Step 2: List the lengths of all sides. Length = 20 cm, Width = 15 cm. Since it's a rectangle, the opposite sides are equal, so we have 20 cm, 15 cm, 20 cm, 15 cm.
---Step 3: Add all the side lengths together. Perimeter = 20 cm + 15 cm + 20 cm + 15 cm.
---Step 4: Calculate the sum. Perimeter = 70 cm.
---Answer: The perimeter of the photo frame is 70 cm.

Why It Matters

Understanding perimeter is crucial for many real-world tasks, from designing a new building to planning a garden. Architects use it to calculate fencing needed, and civil engineers use it for road construction. It's a foundational concept in geometry that helps in understanding space and measurement.

Common Mistakes

MISTAKE: Multiplying all sides instead of adding them. | CORRECTION: Perimeter is always the sum of the lengths of all sides. Multiplication is used for area, not perimeter.

MISTAKE: Forgetting to add all sides, especially for irregular shapes or when some sides aren't explicitly given. | CORRECTION: Always ensure you have accounted for every single boundary line of the shape and added its length to the total.

MISTAKE: Mixing up units, like adding meters and centimeters directly. | CORRECTION: Before adding, convert all lengths to the same unit (e.g., all to meters or all to centimeters) to get a correct total.

Practice Questions

Try It Yourself

QUESTION: A square carrom board has a side length of 70 cm. What is its perimeter? | ANSWER: 280 cm

QUESTION: Your mom wants to put lace around a triangular table mat. The sides of the mat are 30 cm, 40 cm, and 50 cm. How much lace does she need? | ANSWER: 120 cm

QUESTION: A gardener wants to fence a rectangular flower bed that is 12 meters long and 8 meters wide. The fencing costs ₹50 per meter. What will be the total cost of fencing? | ANSWER: ₹2000

MCQ

Quick Quiz

What is the perimeter of a rectangle with length 10 cm and width 5 cm?

50 cm

15 cm

30 cm

25 cm

The Correct Answer Is:

The perimeter of a rectangle is 2 * (length + width). So, 2 * (10 cm + 5 cm) = 2 * 15 cm = 30 cm. Options A and B are incorrect as they represent area and sum of only two sides respectively.

Real World Connection

In the Real World

When a 'mistri' (mason) builds a house, he uses perimeter to figure out how much boundary wall or fencing is needed for the plot of land. Similarly, a tailor uses perimeter to measure the length of trim or 'gota' needed to go around the edges of a saree or dress.

Key Vocabulary

Key Terms

PERIMETER: The total distance around the outside of a shape | LENGTH: The measurement of something from end to end | WIDTH: The measurement of something from side to side | SHAPE: The outline of an area or figure

What's Next

What to Learn Next

Now that you understand perimeter, you're ready to learn about 'Area'. Area tells us the space inside a 2D shape, which is another crucial concept in geometry and builds directly on your understanding of dimensions.