+118608 If you think about and understand this you will see that the cos(angle) is given by the x value on the unit circle. 2pi is one complete revolution. The x value at 0 radians is 1
3pi=3pi-2pi=pi radians on the unit circle. The x value at pi on the unit circle is -1
I hope that helps. 
+23246 Because cos has a period of 2π; therefore, ... cos(4π) = cos(2π) = cos(0) = cos(-2π) = cos(-4π) ...
Cos(π) = -1; so ... cos(-5π) = cos(-3π) = cos(-π) = cos(π) = cos(3π) = cos(5π)...+118608 If you think about and understand this you will see that the cos(angle) is given by the x value on the unit circle. 2pi is one complete revolution. The x value at 0 radians is 1
3pi=3pi-2pi=pi radians on the unit circle. The x value at pi on the unit circle is -1
I hope that helps.
