Figure 2. A radian angle , when S is equal to r.

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A radian angle , when S is equal to r.

Figure 2.

(3) If you designate C the circumference of a circle and d the diameter,

you have

C = d

But the diameter is twice the radius, so

C = 2r

C-= 2  r

You know the circumference of a circle represents 360 of rotation, so

360, = 2  r

then

Dividing

r = 57.3 (approximate)

Hence, you see that a radian is numerically very near 57.3.