Converting to degrees, 4π/3 = 4(60°) = 240°
If we draw a 240° angle in standard position, the terminal side lies in Quadrant 3.
The reference angle for 240° is the acute angle with vertex (0,0) with initial side the negative x-axis and the terminal side coinciding with the terminal side of the given angle.
So,the measure of the reference angle is 60°, which, when converted to radians gives the reference number.
Reference number = π/3
Shortcut (draw θ in standard position where 0 < θ < 2π) Then:
if θ lies in Quadrant 1, then reference number = θ
if θ lies in Quadrant 2, then reference number = π - θ
if θ lies in Quadrant 3, then reference number = θ - π
if θ lies in Quadrant 4, then reference angle = 2π - θ
