To start the applet right click the slider and activate the animation. 

t = 0 (slider from -5 to 5, step 0.01)
a: RandomUniform[5, 10] + (0 * t)
b: RandomUniform[3, 6] + (0 * t)
A: (a, b)
B = (5, 3)
C = (10, 3)
D = (5, 6)
E = (10, 6)
c = Segment(B, C)
d = Segment(C, E)
e = Segment(E, D)
f = Segment(D, B)