0＜x＜1
设x=sinA^2,则1-x=cosA^2
y=1/x+4/(1-x)
=1/sinA^2+4/cosA^2
=1+ctgA^2+4+4tanA^2
=5+(ctgA^2+4tanA^2)>=5+2*√(ctgA^2*4tanA^2)=5+4=9
等号成立时ctgA^2=4tanA^2,=>tanA=1/2=>x=1/5
因此y的最小值是9