# The Perimeter of L Shapes

When teaching perimeter, there are many, many resources at your disposal. It’s in most schemes of work somewhere for all ages, and as such many people have made worksheets and the like.

I have a bug bear with perimeter worksheets though. There is almost always a complete excess of information provided, often making questions more difficult than they need to be. This is particularly true when students need to find the perimeter of L-shapes (because “important skill”).

Now, when teaching area of a triangle, an excess of information is really useful, as it helps pick out the students who don’t know what they’re doing. Similarly, removing a lot of the information with L-Shape perimeter questions, the opposite is true.

Look at the examples below:

For each of the above, you can solve them with about half of the information given. Furthermore, by removing the excess information, you’re showing students something more important – that deduction is possible, and is useful.

Obviously this does not apply to every shape. Take this example:

This wouldn’t work quite in the same way, as the 6cm cannot be deduced if it is removed. However, both the “3cm” can happily disappear.

This question popped up in my lesson last week:

Now, to me, this question is actually (pointlessly) more difficult with the 3cm height included. It infers that you may need to either dissect the shape, or start with the expression

h+(h-3) +3 +20+3+(20-3) = 72

rather than just using

2h + 40 = 72

Perhaps the question is being clever and expecting students to *see* the route with fewer calculations by ignoring some measurements, but the amount of space provided for an answer (not shown above) suggests not.

Silly perimeter questions.

## 5 thoughts on “The Perimeter of L Shapes”

1. Mark Wilson says:

Wow, this is really great, I’d never thought of it like this, I think you are totally right and I can’t wait to try it with some pupils, sometimes I wish I taught more ks3 but I will give it a go with Y11

2. Dumb question : Does anyone draw the containing rectangle, which has the same perimeter, and use it for the (basically the same) calculations ?

• Good question. I think the majority mindlessly add up everything they can see

3. Perhaps not as mindless as one might think ! Kid reads problem. Thinks “If I wanted to know the perimeter of this shape I would have measured all the sides” – and purposely gives a dumb answer to what is seen as a dumb question.