A) KẺ AA'  DC = A'; EE'  DC = E'; BB'  DC = B'  12(AA' +...

6. a) Kẻ AA'  DC = A'; EE'  DC = E'; BB'  DC = B'  12(AA' + BB') S

EDC

= 12DC.EE' 1 ' 'A A B B  