Mr Barton’s Maths Notes. Shape and Space 2. Polygons. www.mrbartonmaths.com. One of Mr Barton’s Top 10 Maths Jokes What did the pirate (who was also a very keen mathematician) say when his parrot flew away?... “ Poly-gon !”… you can’t beat a maths joke, hey?... anyway…. What are Polygons?
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Shape and Space
What did the pirate (who was also a very keen mathematician) say when his parrot flew away?... “Poly-gon!”… you can’t beat a maths joke, hey?... anyway…
What are Polygons?
A Polygon is any closed shape which has three or more sides.
All their sides are the same length, and all their angles are the same size
e.g. squares, equilateral triangles, regular octagons…
You’ve guessed it… these do not have equal length sides and angles
Rectangle, kites and trapeziums are an irregular polygons, but so too are shapes like this:
Two types of Polygons that you must be especially clued up about are quadrilaterals and triangles
There are 4 types of triangles you need to be on the look-out for and you must know the properties of (what is special about) each of them
A Quadrilateral is any four-sided shape. There are lots of quadrilaterals flying around, and it is important that you know the properties of each… so here they are!
Notice: Each of the four shapes above are very similar… in fact, they are all just special types of parallelograms! See how they each have two pairs of parallel sides… and then it just certain other properties that make them different shapes!
As soon as you get above 4 sides, the names of the polygons start to get a bit weird. Here are some of the main ones you should learn.
Notice: Each of the shapes below are regular polygons as all the sides and angles are the same… but any 8 sided shape is still an octagon, it may just be an irregular one!
Heptagon / Heptagon
An interior angle is any angle inside the polygon
If we are told the number of sides a polygon has, we can work out the total sum of all the interior angles using this little formula:
Sum of all interior angles = (Number of sides of polygon – 2) x 180
Well, it’s all to do with triangles…
We know that the sum of the interior angles of any triangle is 1800, right?
Well… we can split any polygon up into triangles, like this…
And there will always be 2 fewer triangles than there are sides!
For Regular Polygons
Because all angles are equal in regular polygons, you can work out the size of each interior angle like this:
Size of each interior angle = Sum of all interior angles ÷ Number of sides
An exterior angle is an angle outside the polygon made by extending one of the sides…
And here is the fact!
Sum of all exterior angles = 3600
Well, if you keep moving around the polygon, extending the sides and measuring each exterior angle, by the time you get back to where you started you have made… a circle! Which, as we all know, contains 3600
For Regular Polygons
If all interior angles are equal for regular polygons, then all exterior angles are equal too, so to work out the size of each one, we do this…
Size of each exterior angle = 3600 ÷ Number of sides
Note: If you know the sizes of the exterior angles of a regular polygon, then you can also work out the sizes of the interiors by remembering that angles on a straight line add up to 1800
Size of each interior angle = 1800 – Size of each exterior angle
Using the formulae we have talked about, it is possible to work out pretty much any angle fact about any size polygon. Have a practice to make sure you can get the numbers in this table…