Sin 90 degrees is equal to one. This degree value can also be expressed in radians as sin(?/2) = 1. This value of the sine function corresponds to one-fourth of the complete arc distance along the unit circle.
Radians are a unit of angular measurement that describe an angle in terms of the length of a corresponding arc on the unit circle. Radians are found using the length of the arc divided by the radius of the arc. There are 360 degrees in a complete unit circle, which is equal to 2? radians. This means that 90 degrees is equal to ?/2 radians and this gives sin(?/2) a value of 1.