P点会在B--D上任意位置.可以看P从B移动到D的时候EF的长度会产生变化.P在B点,EF = BC,PBE重合,FC重合,在D点反之,EF =DC.最短的是P在中点,EF = 1/2 BD.EF = PA = PC.PA 垂直于 EF.从AP作延长线到EF相交于点G.∠PEG = ∠EFC∠EPG = ∠BAP = ∠PCE = ∠FEC∠PGE = 90°.
