Cho tu dien $ABCD$.Goi $M$ thuoc canh $AB$ sao cho $AB=3AM,N,K$ lan luot la trung diem $AC,AD;MN$ giao $BC=E,MK$ giao $BD=F;I$ doi xung voi vs $M$ qua $N,J$ doi xung voi $M$ qua $K$.
Chung minh :
$1/ CD$ song song $EF$
$2/ CD$ songsong $IJ$
$3/CI$ songsong $AB$