∫ The straight lines l∑BC, BA, DQ and CN intersect in F, ∴QA/NQ=BD/DC, or QA/Na = BD/BC...①;

Similarly, CA, DP and BM meet at E,∴PM/AP=BD/DC, or AM/AP = BC/DC...②;

* cn, DA and BM meet in H, ∴ AM/NA = BD/DC...③,

①× ② (QA/NA)*(AM/AP)=BD/DC, or (AQ/AP)*(AM/NA)=BD/DC,

Compared with ③, AQ/AP= 1, that is, AP=AQ.