# Perimeter question

Discussion in 'Mathematics' started by redredrobin, Feb 11, 2012.

1. ### redredrobin

If you have a shape with the middle cut out, would the perimeter include the sides of the cut out section?

2. ### redredrobin

If you have a shape with the middle cut out, would the perimeter include the sides of the cut out section?

3. ### DMNew commenter

Good question.
The shape has two perimeters, an inner perimeter and an outer perimeter. If the question asks for the perimeter and does not clearly identify which one, I would imagine they are looking for the outer perimeter. I would not add the two perimeters together.

4. ### redredrobin

Thank you - that's what we'll go with!

5. ### SC00BY

So DM, are you saying the perimeter of an annulus is the same as the outside circle? I thought the perimeter would be the total of both the outside and inside one.

6. ### DMNew commenter

Depends on the audience. The OP is a KS1 teacher and I think the message that needs to be shared is that the perimeter is the exterior boundary of a shape.
(Between you and me, the formula for the perimeter of an annulus is often given as 4 pi x the mean radius).

7. ### SC00BY

If the OP is a KS1 teacher, my advice would be to avoid such shapes and concentrate on shapes without holes. However, if a pupils asks the question regarding a shape with a hole in it, for example a square within a square, my advice would be to give the correct response that the perimeter is the length of the boundary of the shape which includes both interior and exterior ones. This will avoid any confusion later on when the pupil comes across such shapes as the annulus.

8. ### DMNew commenter

Not easy to do if he or she is using a commercial package that defines perimeter as the exterior boundary of a shape and illustrates this point with "holey" examples.

9. ### redredrobin

I am a ks1 teacher but this wasn't for my class. It was my 8 yr old daughter's homework and was indeed a commercial worksheet to find area and perimeter. It got my husband and I into a discussion about what would be mathematically correct. Maybe I'll get her to give measurements for external and internal perimeters so that she knows both are perimeter.