Dung DH vuong goc vs AB suy ra BH=HA=CD=$\frac{1}{2}AD$=>$\widehat{HDA}=30^{o}$Goi giao cua AD va BC la E. Suy ra D la Trung diem cua AE va $\widehat{BEA}=\widehat{HDA}=30^{o}$
Lap pt duong thang AD di qua N(-2,0) va tao vs BC goc $30^{o}$
Tu pt AD lap dk tim ra E.
Tim dk E ta tim dk D thong qua $\overrightarrow{ED}=\frac{2}{3}\overrightarrow{EM}$
Tim dk D se lap dk pt duong DC qua D vuong BC.
Giao cua DC tim dk va BC ta tim dk C.