Functions that make use of angles (like sin and cos, for example) have to know if the angles they are working with are in degrees or radians. If the calculator is in degree mode the functions automatically assume that numbers they are given are in degrees; if it is in radian mode the functions automatically assume that numbers they are given are in radians.
eg degree mode: cos(5) = 0.9962 the 5 is assumed to be degrees
radian mode: cos(5) = 0.2837 the 5 is assumed to be radians
There are 360 degrees in a revolution and
There are 2π radians in a revolution.
Angles can be measured in either degrees or radians.
So if I am in school and requires a calculator, what mode should I use?
If you are not studying a level maths then always make sure you're in degrees, no radian stuff comes up in gcse, in a level maths only questions which specify radian use will you need to change your calculator out of degree mode, so i would say always keep it in degree mode and only change it when needed