Cho  khi đó ta có:

\[\frac{{DE}}{{HK}} = \frac{{DF}}{{KI}}.\]

\[\frac{{DE}}{{HK}} = \frac{{EF}}{{HI}}.\]

\[\frac{{EF}}{{KI}} = \frac{{DF}}{{HI}}.\]

\[\frac{{DF}}{{HI}} = \frac{{EF}}{{HK}}.\]