# Cross Perimeter

65cm

70cm

Calculate the distance around this shape. All angles are right angles but the diagram is not drawn to scale.

Show Hint

## A Mathematics Lesson Starter Of The Day

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Topics: Starter | Geometry | Mensuration

• Mr Wheldon, Springwood Heath
•
• Shayne got this in 5 seconds!!!
Well done.
• Mandy Smith, Anderton Park
•
[Mandy, the answers are further down this page but only appear for teachers when they are logged in - Transum]
• Sergio R., Colombia.
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• It is really nice to comment and notice that you can reduce the area an the perimeter will be the same, in the infinite you will have a cross without area.
• Transum,
•
• That's a very interesting observation Sergio. What we really need is an animation to show that. It is not intuitive for most people to realise that you can increase or decrease the area of a shape while keeping the perimeter the same.

There was a similar Starter last October called Stair Perimeter. Underneath the diagram is a button labelled 'Show Hint'. Click the button to show an animation that should make the situation clear.

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## Extension

If all of the vertical edges are the same length and all of the horizontal edges are the same length, what is the area of the shape?

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 Teacher, do your students have access to computers?Do they have iPads or Laptops in Lessons? Whether your students each have a TabletPC, a Surface or a Mac, this activity lends itself to eLearning (Engaged Learning).

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Here is the URL which will take them to a related student activity.

Transum.org/go/?to=areacomposite