I read this in a maths magazine once, but never quite got the answer.
You are given a right-angled isosceles triangle. You need to draw in straight lines to form acute triangles, until you are left with only acute angled triangles. (By triangles I am not counting triangles that can be formed by two or more adjacent small triangles together.)
I read that the answer is that you'll be left with 7 acute-angled triangles.
Can anyone prove this to me?