An angle of 2π rads us 1 revolution so 6π is 3 revolutions. cos(0) = 1, cos(2π) = 1, cos(6π) = 1.
An angle of π/2 is a quarter turn or right angle anticlockwise so an angle of – π/2 is a quarter turn or right angle clockwise. Therefore, 11π/2 is 11 right angles clockwise which is 2 3/4 full turns. Sin(π/2) = 1, sin(-π/2) = -1, cos(-11 π/2) = sin(π/2) = 1