To get the length of the other diagonal, DB, we'll use this "formula" to find the measure of angles DAC and CAB :
Cos θ = [u(dot)v] /( llull * llvll) Where u and v are vectors and
[u(dot)v] - the dot product of u and v and
llull = length of u and
llvll = length of v
So, taking AD and AC as vectors
AD =<5.4, 39.7> AC = <40.6, 40.2> AD(dot)AC = 1815.18
llADll = 40 llACll = 57.1
cos-1 (1815.18)/(40*57.1) = m< DAC =37.37
And again, taking AB and AC as vectors
AB = <40.6 ,0> AC <40.6, 42.7> AB(dot)AC = 1648.36
llABll = 40.6 llACll = 57.1
cos-1 (1648.36)/(40.6*57.1) = m< CAB = 43.8
m< DAB = m< DAC + m< CAB = 37.37 + 44.7 = 82.07
And using the Law of Cosines, DB = SQRT (40^2 + 40.6^2 - 2(40)(40.6)cos(82.07)) = 52.92 ≈ 53
And that's close enough for me!!