如图,已知正方形ABCD的边长a,E是对角线BD上一点,BE=a,P是EC上任意一点,PM垂直BD于M,PN垂直BC于N

如图,已知正方形ABCD的边长a,E是对角线BD上一点,BE=a,P是EC上任意一点,PM垂直BD于M,PN垂直BC于N,求PM+PN的值
http://image.baidu.com/i?ct=503316480&z=0&tn=baiduimagedetail&word=%C8%E7%CD%BC%2C%D2%D1%D6%AA%D5%FD%B7%BD%D0%CEABCD%B5%C4%B1%DF%B3%A4a%2CE%CA%C7%B6%D4%BD%C7%CF%DFBD%C9%CF%D2%BB%B5%E3%2CBE%3Da%2CP%CA%C7EC%C9%CF%C8%CE%D2%E2%D2%BB%B5%E3%2CPM%B4%B9%D6%B1BD&in=15883&cl=2&cm=1&sc=0&lm=-1&pn=1&rn=1&di=9861271650&ln=2000&fr=&ic=0&s=0&se=1#pn1 图片