连接AC、CF因ABCD、CEFG为正方形,则角ACD=角FCD=角BAC=45度,角ABP=90度则角ACF=角ACD+角FCD=90度因角FAP=45度=角CAF+角CAP,角BAC=45度=角CAP+角BAP则角BAP=角CAF又角ABP=角ACF=45度则三角形ABP相似ACF则AC/AB=AF/AP又AC=根2*AB所以AF=根2*AP