Angles – The seed of Trigonometry

Angle in degrees or radians? Which one to choose? 

Why do we express angles in terms of fractions or multiples of π i.e. 30°, 90°, 180°, 360°… as  π/6, π/2, π, 2π … ?

Why does a circle have 360°?  Why is 360° = 2π?

What is behind the periodicity of trigonometric functions?

For answers, read on…