A PARALLELOGRAM
A quadrilateral with both pair of opposite sides parallel.

PROPERTIES OF A PARALLOGRAM
1) Opposite sides are equal
2) Opposite angles are equal
3) Diagonals bisect each other
4) Adjacent angles are supplementary.

a = 10 (slider from 4 to 10, step 1)
A = (-2.032513791407211, -2.0617930592981786)
B = Point(Circle[A, abs(a)])
b = Segment(A, B)
D = (0.46600212678384345, 3.7680774164809545)
c_1 = Line(D, b)
d = Line(D, A)
e = Line(B, d)
C = Intersect(c_1, e)
poly1, a_1, b_1, c, d_1 = Polygon(A, B, C, D)
distanceAB = Distance(A, B)
TextAB: Name[A] + (Name[B]) + " = " + distanceAB
distanceBC = Distance(B, C)
TextBC: Name[B] + (Name[C]) + " = " + distanceBC
distanceCD = Distance(C, D)
TextCD: Name[C] + (Name[D]) + " = " + distanceCD
distanceDA = Distance(D, A)
TextDA: Name[D] + (Name[A]) + " = " + distanceDA
α = Angle(B, A, D)
β = Angle(A, D, C)
γ = Angle(D, C, B)
δ = Angle(C, B, A)
f = Segment(A, C)
g = Segment(D, B)
E = Intersect(g, f)
h = Segment(A, E)
i = Segment(E, C)
j = Segment(D, E)
k = Segment(E, B)
distanceAE = Distance(A, E)
TextAE: Name[A] + (Name[E]) + " = " + distanceAE
distanceEC = Distance(E, C)
TextEC: Name[E] + (Name[C]) + " = " + distanceEC
distanceDE = Distance(D, E)
TextDE: Name[D] + (Name[E]) + " = " + distanceDE
distanceEB = Distance(E, B)
TextEB: Name[E] + (Name[B]) + " = " + distanceEB
text3: "OPPOSITE SIDES
AB = CD =  " + a + "
AD = CB =" + b_1 + ""
text4: "OPPOSITE ANGLES
∠A = ∠C =" + α + " 
∠B = ∠D =" + β + ""
text5: "DIAGONALS
DE = EB =  " + j + "
AE = EC = " + i + " "
θ: α + β
text6: "ADJACENT ANGLES
∠A + ∠B =  " + α + " +  " + β + "= " + θ + "
∠B + ∠C = " + γ + "+ " + δ + " = " + θ + ""