Ta có:
`AD^2 + DC^2 = ((a\sqrt{3})/4)^2 = (3a^2)/16`
`=> 2AD^2 = (3a^2)/16`
`=> AD^2 = (3a^2)/32`
`=> AD = sqrt{(3a^2)/32} = (a\sqrt{6})/8`
`|vec{AC} + vec{BD}|`
`= |vec{AB} + vec{AD} + vec{BA} + vec{BC}|`
`= |2vec{AD}|`
`= 2.AD`
`= 2.(a\sqrt{6})/8`
`= (a\sqrt{6})/4`