Perimeters of polygons
Solve problems involving perimeters of polygons: find perimeter from side lengths, find an unknown side length, and explore rectangles with same perimeter but different areas (or vice versa)
What a learner can do afterwards
- Calculate the perimeter of a rectangle with sides 8 cm and 5 cm
- Find the missing side of a pentagon with perimeter 30 cm and four known sides
- Draw two rectangles both with perimeter 24 cm but different areas
The lesson
You already know how to measure a perimeter by tracing around a shape. Now you can find it just by adding up side lengths you're given, with no ruler needed. Add the length of every side of a polygon, and that total is the perimeter.
A pentagon's perimeter is 30 cm. Four of its sides measure 4 cm, 5 cm, 7 cm, and 8 cm, and the fifth is hidden. Add the known sides: 4 + 5 + 7 + 8 = 24 cm. Subtract that from the total: 30 - 24 = 6 cm. The missing side is 6 cm.
When you're hunting for a missing side, add up the sides you know first, then subtract from the whole perimeter. And don't assume two shapes are the same just because their perimeters match.
Add every side for the perimeter, subtract known sides from the total to find one that's missing, and remember that equal perimeters can hold different amounts of space.
Watch it
Where it sits
Where this leads
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.