角の大きさ

We measure angles in degrees (°). A full turn all the way around is $360^{\circ }$. The most important one is the right angle, which is exactly $90^{\circ }$. The corner of a piece of paper or a book is a right angle. A small square drawn in the corner is the sign for a right angle.

We measure the size of an angle with a protractor. First place the centre of the protractor exactly on the vertex (the point of the angle) and line the $0$ line up along one arm. Then read the mark that the other arm passes through. A protractor has two scales, inner and outer, so read the one that starts from the $0$ you lined up. In Cambridge Primary Stage 5, the aim is to read angles correctly to about the nearest $5^{\circ }$.

Taking the right angle as $90^{\circ }$, we can sort angles like this. An acute angle is smaller than $90^{\circ }$; it looks sharp and narrow. An obtuse angle is between $90^{\circ }$ and $180^{\circ }$; it looks wide and open. A straight line is exactly $180^{\circ }$ — perfectly flat, the same as two right angles.

Angles that sit together on a straight line always add up to $180^{\circ }$. So if you know one of them, you can find the other by subtracting from $180^{\circ }$. For example, if there is a $110^{\circ }$ angle on a straight line, the angle left over is $180^{\circ }-110^{\circ }=70^{\circ }$.