The **radian**, denoted by the symbol rad, is the SI unit of angles.

Consider an angle subtended by an arc of a circle. The magnitude of that angle *θ*, in radian, is simply the ratio of the arc length *s* to the radius *r*.

Based on the definition, 1 radian would be the angle subtended by a circular arc same length as the radius, which turns out to be about 60°. To be more exact, (8 d.p.).

Since the circumference of a circle would subtend an angle of 360°, 360° must correspond to . By simple proportion, we can work out the other “important” angles.

Personally, I “memorise” to help me convert any angle between ° and rad by using simple proportion. E.g.

and

.

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**Concept Test**

1001

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