若圓被軸截得弦所對(duì)圓心角為，則實(shí)數(shù)=        

 

 若圓被軸截得弦所對(duì)圓心角為，則實(shí)數(shù)=          

若圓C：x²+y²-2ax-2y+a²=0（a為常數(shù)）被y軸截得弦所對(duì)圓心角為

π

2

，則實(shí)數(shù)a=．

若圓C：x²+y²-2ax-2y+a²=0（a為常數(shù)）被y軸截得弦所對(duì)圓心角為，則實(shí)數(shù)a=    ．