Mình cũng tính ra$ \frac{2a}{\sqrt{10}}$
SO =$ \frac{a\sqrt 2}{2\sqrt 3}$
d(AM,CD) = d (CD, (SAB)) = d (C, (SAB)) = 2 d(O, (SAB)) = 2$\frac{(\frac{a\sqrt 2}{2\sqrt 3}) .(\frac{a}{2})} {\sqrt{(\frac{a\sqrt 2}{2\sqrt 3})^2 + (\frac{a}{2})^2}} = \frac{a\sqrt{10}}{5}$