3.tanPAE=PE/PA=1/2=>由相似型的性质：PE/PA = PC/AB = CE/BP = Y/X = 1/2点P在边BC上时,=>Y=1/4X(5-X)=>X=5/3点P在边BC延长线上时,=>BP=X,PC=X-5=>(X-5)/4=1/2X=7
