The above shows a unit circle and the angle h shrinking to 0.

The length of the red arc is h measured in radians
The length of the blue vertical line is sin(h)

As h goes to zero it can be seen
sin(h) and h become closer and closer to the same length.
The limit as h approaches 0 of sin(h)/h = 1.

Other trigonometry pages by Hop:
What is sine and cosine?
Measuring an angle in radians, generating a sine wave
sin(a+b) and cos(a+b)

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