How many different kinds of triangle are there?

How many different kinds of triangle are there?

Four! Right? There are equilateral triangles, isoceles triangles, right-angled triangles and scalene?

But what happens if you cut a square in half corner to corner? What kind of triangle is that?

Well it's right-angled and it is also isoceles! So there are different kinds of isoceles triangle. Some are right-angled. Some are not!

What other kinds of isoceles triangle can you have? Can you have one only with acute angles? Can you have one with an obtuse angle?

What about other triangles? Can you have a triangle with two right angles? What about two obtuse angles? Why?

There's a lot more here to investigate than first met the eye...

Ages 8-12    Printable materials (pdf) - 3 pages of teachers notes and 7 pupil copymasters.