Reminder, obtuse and reflex angles

Mathematicians can never resist the temptation to put things into categories, thus giving them the opportunity to invent a new collection of terms. Angles are classified, in order of size, as: acute, right, obtuse, straight or reflex. An acute angle is an angle less than a right angle. An obtuse angle is an angle between a right angle and a straight angle. A reflex angle is an angle greater than a straight angle, but less than a whole turn.

Pupils can cut out pictures from magazines, mark angles on them and display them in sets as acute, right, obtuse, straight and reflex.

How can you see that the three angles in a triangle add up to 180 degrees?A popular way of seeing this property of the angles of a triangle is to draw a triangle on paper, mark the angles, tear off the three corners and fit them together to discover that together they form a straight angle, or two right angles (180º). What about the sum of the angles in shapes with more than three sides? The four corners of any quadrilateral (a plane shape with four straight sides) can be torn off and fitted together in the same way as was done with a triangle. It is pleasing to discover this way that they always fit together to make a whole turn, or four right angles (360º).