In the context of calculus we always use radian measure for angles because it makes the constants in certain important limits turn out to be 1.
Notice that this simulataneously defines both 
and 
and makes the identity 
obvious, because the point 
is on the unit circle.
Certain key values can also be determined with little effort, noting that a whole unit circle has circumference 
, so a quarter of a circle is 
:
