Polygon vocab: it’s all Greek to me.

February 5, 2008

Another cross-post to FlyingColoursMaths.co.uk.

Sadly, the maths GCSE seems to be almost as much about learning vocabulary you’re never going to need to use again as it is about learning to do maths (seriously, I’ve not had any use for the word ‘congruent’ in more than a decade of serious science, much of which involved geometry – it’s like the map of the cat). But, as with fractions, they have to be learnt, and, as with pizza, I’m here to make life easier.

Triangle is easy. Tri- means three (tricycle, triathlon) and -angle means corner. -Gon means corner too, which is where the word trigonometry comes from. Tri-gon-ometry: three-corner-measuring. There are three main types of triangle: the equilateral, (equal sides – in English, lateral is another word for sideways), which has three equal sides; the isosceles, which has two sides the same (iso- means the same, and -sceles comes from skelos, meaning leg); and the scalene, which has three different lengths of side (from skalenos, meaning rough. Apparently).

There are also right triangles, which contain a right angle (90º) and are either isosceles (if the other angles are 45º) or scalene (otherwise). The angles inside any triangle add up to 180º – check that it works for an isosceles right triangle.

Four-sided shapes have many more varieties. There’s the square and rectangle, of course, both of which have right angles at all four corners. Then there’s the parallelogram, which is what you get if you squish a rectangle so that opposite sides are parallel, but the angles aren’t 90º any more. A rhombus is a the same thing, but starting from a square. Then there’s the trapezium, which looks a bit like a trapeze: it’s got a bar, parallel to the moorings at the top, and the other two sides can be at any angle. There’s also the kite, which is a lot trickier to describe than you’d think. Take a pair of lines the same length joined together. Take another pair of lines the same length (not necessarily the same length as the first pair) and join them together too. Then join the loose ends of the two pairs together and bang, you have a kite.

A polygon (poly- = many, -gon = corner) is a shape with many corners (three or more). After four-sided shapes, polygons are always called (something-)gons. A pentagon has five corners (penta- unsurprisingly, means five); a hexagon six. Then heptagon (7), octagon (8), nonagon (9) and decagon (10). If all the sides are the same length, you can call them equilateral (again) or regular.

I think those are the main vocab words you need to know for polygons. If I’ve missed any, let me know.