y=(sina)/(1＋cosa)
=[2sin(a/2)cos(a/2)]/[1＋2cos²(a/2)－1]
=[2sin(a/2)cos(a/2)]/[2cos²(a/2)]
=[sin(a/2)]/[cos(a/2)]
=tan(a/2)
周期是π/(1/2)=2π