Perimeter and Area of Irregular Shapes

Finding Parallel Sides

The first thing my students practice is finding the set of parallel sides. Doing this will help the students calculate the lengths of missing sides as well as reinforce the concept of parallel sides.

To break this down even further, the students set up equations. This helps them with identifying the two short lengths that make the long length of each parallel side.

Finding the Perimeter

Build off finding the set of parallel sides to find the missing lengths.

Perimeter is the length or distance that goes around. Instead of adding all of the individual sides together (which of course students could do), I encourage my students to use the formula of finding the perimeter of a rectangle. Help the students make the connection that the long sides make up the length and the width.

After helping students make the connection of the rectangle, they use the formula p = (2xl) + (2xw). Using the formula not only reinforces the skill but helps reduce computation errors of adding up a whole set of numbers.

Finding the Area

Once again, I always start by building off of finding the set of parallel sides and calculating any missing side lengths.

Since area is the space inside a shape, break the shape into two separate shapes.



Find the area of the two separate quadrilaterals using the formula A = l x w.

A: 7 x 3 = 21 cm squared

B: 10 x 5 = 50 cm squared

Combine the areas of the two separate quadrilaterals to find the total area of the irregular shape.

Is there a specific strategy you use in your classroom to help with perimeter and area of irregular shapes? Let me know!

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