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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 $\small BD = \dfrac{\overline{AB}×\sin(\beta)}{\sin(\alpha)} = \dfrac{12\sqrt2×\sin(75°)}{\sin(60°} = 12+4\sqrt3 \quad (= 4(\sqrt3 +3)).$