Perimeter

Last lesson, you found the area of a figure by counting the square units it covers. Today, we will find the perimeter of a figure by going all the way round it. Lin cuts out the L-shaped card shown. Find its perimeter. The perimeter is the total length all the way round the figure. Start at the top left corner. This side is 8 centimetres. This side is 2 centimetres. This side goes in, 3 centimetres. This side is 4 centimetres. It goes in too, but it is still part of the way round. This side is 5 centimetres. This side is 6 centimetres. We are back where we started, so we have gone all the way round once. 8 plus 2 plus 3 plus 4 plus 5 plus 6 is 28 centimetres. To find a perimeter, go all the way round and add every side.

Lin draws a square card. Each side is 6 centimetres long. Find its perimeter. Try it first. Pause if you like. A square has 4 sides of the same length. This side is 6 centimetres. This side is 6 centimetres too. We are back where we started, so we have gone all the way round once. 6 plus 6 plus 6 plus 6 is 24 centimetres.

The figure shows Lin's stepped card. Find its perimeter. Try it first. Pause if you like. The top is 10 centimetres, but the bottom is only 6 centimetres. The step makes up the rest of the top. 10 minus 6 is 4 centimetres. So the step is 4 centimetres long. This side is 10 centimetres. This side is 3 centimetres. This side is 4 centimetres. This side goes in, 5 centimetres. This side is 6 centimetres. This side is 8 centimetres. We are back where we started, so we have gone all the way round once. 10 plus 3 plus 4 plus 5 plus 6 plus 8 is 36 centimetres. Perimeter is the total length all the way round a figure. Add every side, even the sides that go in.