Kẻ HE\bot A'B'
S_{B'A'D'}=\dfrac{3a\sqrt3}4\Rightarrow d(D',A'B')=\dfrac{3a}2\Rightarrow HE=\dfrac{1}2d(D',A'B')=\dfrac{3a}4
((ABCD),(CDD'C'))=((A'B'C'D'),(ABB'A'))=\widehat{BEH}
\cos \widehat{BEH}=\dfrac{EH}{BE}=\dfrac{\sqrt{21}}7\Rightarrow EB=\dfrac{a\sqrt{21}}{4}\Rightarrow...