∵∠PAB=∠PDC ∠PBA=∠PCD∴△ABP∽△DCP∴AB/CD=AP/DP=3/1=3∵AB是圆O的直径∴∠ADP=90°sin∠DAP=DP/AP=1/3cos∠DAP=√[1-(sin∠DAP)^2]=2√2/3
