In 1946, ErdÐ â s himself posed a riddle about what are called “isosceles sets”: How many points can you place in d -dimensional space such that any three of them form an isosceles triangle?

An isosceles triangle with one angle greater than 90° is an obtuse-angled triangle. An isosceles triangle with angles of 100°, 40° and 40° is an obtuse-angled triangle.

Students are often unsure why angles in the same segment of a circle have to be equal. Colin Foster explains how to make it ...

Working with Sketch 2 Project Sketch 2 on the screen and ask the students to explore which coloured triangles could fit into the given triangle outlines. For this sketch, the eyes and nose outlines ...