 Perimeter II

Lesson

Finding the perimeter

For any polygon, or 2D shape, we can add up the length of every side to find the perimeter of the shape. Rectangles and squares have special features that mean we don't need to know the length of every side, since some sides are the same length.

Special quadrilaterals also have features that are really useful to find the perimeter. Let's see how we can use them to find the perimeter of a parallelogram, kite, rhombus and trapezium.

Worked Example

Question 1

Find the perimeter of the trapezium shown. Using the perimeter to find length

When we have shapes, such as special quadrilaterals, and we know the perimeter, we can often find the missing length of some sides. Knowing the things that make our shapes special helps us with this, so let's see how we can use the perimeter to find the length of some sides. It's just like finding the missing value from any polygon, except we may not need to know the length of as many sides in some cases!

Worked Examples

Question 1

Find the length, in centimetres, of the shortest side of the kite shown. Use $s$s as the length of the side. Question 2

Find the length of each side of a rhombus if its perimeter is $40$40 mm.

Remember!

The way we calculate the perimeter, or work out the length of sides, doesn't change with special quadrilaterals. With some shapes, we just have some extra things to help us, so identifying those things can make the process a little easier.

Outcomes

GM4-3

Use side or edge lengths to find the perimeters and areas of rectangles, parallelograms, and triangles and the volumes of cuboids