PE⊥PA
∠APE=90
∠APB+∠CPE=90
又因为正方形ABCD
∠ABP=∠ECP=90
∠APB+∠PAB=90
∠CPE=∠PAB
△ABP∽△PEC
又因为BP=5,BC=AB=12
PC=7
CP/AB=EC/BP
EC=35/12
DE=CD-EC=109/12