将△APD逆时针旋转90° ,
此时AB与BC重合,
设D是旋转后P,连结PD,交BC于E
∴△ABD≌△CBE
∴∠BAD = ∠BCE
∵∠BEA = ∠DEC
∴∠ABC = ∠EDC
∴∠EDC = 90°
∴△PDC是Rt△
∵CD = AP = √5,PC = 5
∴PD = 2√5
∵PB = DB,∠PBD = 90°
∴△BPD是等腰Rt△
∴PB = PD/√2= √10