sin 60° / 12√2 = sin 75° / BD
BD = 12√2 * sin 75° / sin 60°
BD = 12√2*(√2 +√6)/(4*√3 /2)
BD = 12√2*(√2 +√6)*2*√3/12...12 si semplifica
BD = 4√3+2*6
BD = 4(√3+3)
========================================================
Angolo alla base $\small \widehat{ABD} = \widehat{BDC} = \gamma = 45°;$
angolo alla base $\small \widehat{BAD} = \beta = 75°;$
angolo al vertice $\small \widehat{ADB} = \alpha = 180-(\beta+\gamma) = 180-(75+45) = 180-120 = 60°;$
per cui:
diagonale $\small BD = \dfrac{\overline{AB}×\sin(\beta)}{\sin(\alpha)} = \dfrac{12\sqrt2×\sin(75°)}{\sin(60°} = 12+4\sqrt3 \quad (= 4(\sqrt3 +3)).$