Angles

For grown-ups: this lesson in words

Years 3 to 6 · Maths

Angles: I can name, measure and compare angles, and find missing angles.

1. An angle is made where two straight lines meet. It measures how much one line has turned from the other.

   Here is a square. Each of its 4 corners is a right angle: a square corner.

   Here is a right angle on its own. A little square marks it. We write its size as 90°.

   Picture: 2 shapes. 1: Square: a square with its 4 right angles marked. 2: Right angle: a right angle of 90 degrees with its 1 right angle marked.

2. Angles also measure turns. Turning all the way round, back to where you started, is a full turn.

   We measure angles in degrees. The sign for degrees is °. A full turn is 360°.

   A quarter of the circle is shaded: a quarter turn. 360 ÷ 4 = 90, so a quarter turn is 90°, a right angle.

   Picture: A circle cut into 4 equal parts, with 1 part shaded. It is labelled 1/4.

3. Now 2 of the 4 quarters are shaded: two quarter turns.

   2/4 of the circle is shaded. Two quarters make a half, so this is a half turn.

   A half turn is two quarter turns: 90 + 90 = 180. A half turn makes a straight line, 180°.

   Picture: A circle cut into 4 equal parts, with 2 parts shaded. It is labelled 2/4.

4. Four angles. B is a right angle, 90°. A is smaller than a right angle: 45°. We call it acute.

   C is bigger than a right angle but smaller than a straight line: 135°. We call it obtuse.

   D is 240°. It opens wider than a straight line, more than 180°. We call it reflex.

   In order of size, smallest first: A, B, C, D. 45°, 90°, 135°, 240°.

   Picture: 4 shapes. 1: A: an acute angle of 45 degrees. 2: B: a right angle of 90 degrees. 3: C: an obtuse angle of 135 degrees. 4: D: a reflex angle of 240 degrees.

5. Your turn: Here is an angle. Compare it with a right angle, a square corner. Is this angle acute, right or obtuse?

   Picture: An angle smaller than a square corner.