Ta có: `vec(BA^')=vec(CD^')` `=>` `vec(AD^')+vec(BA^')=vec(AD^')+vec(CD^')`

`=>` `(vec(AD^')+vec(BA^'))^2=(AD^')^2+(CD^')^2+2*vec(AD^')*vec(CD^')`

`=2a^2+2a^2+2*AD^'*CD^'*cos(vec(AD^'),vec(CD^'))`

`=4a^2+2*(asqrt2)^2*cos hat(AD^'C)`

`=4a^2+4a^2*cos hat(AD^'C)`

Mà `DeltaAD^'C` là tam giác đều `=>` `hat(AD^'C)=60^o` `=>` `cos hat(AD^'C)=1/2`

`=>` `(vec(AD^')+vec(BA^'))^2=4a^2+2a^2=6a^2`

`=>` `|vec(AD^')+vec(BA^')|=sqrt6a`

`=>` `C`