#1# degree is equal to #pi/180# radians.
#1# radian is equal to #180/pi# degrees.
From here we can see that #pi# radians is #180# degrees.
Remember that negative angles are angles that go clockwise.
#pi# radians is the same as #-pi# radians because going #180# degrees counterclockwise is the same as going #180# degrees clockwise. You can imagine this.
Therefore #-pi# radians is #180# degrees.
For angles in general, #-r# radians is the same angle as #2pi - r# radians. Similarly, #-r# degrees is the same angle as #360-r# degrees. You can do this if you want to work with positive angles.
