y=1/cos²θ+1/(x(1-x)sin²竖隐θ)〉=1/cos²θ+1/(4/sin²θ) {注:余老厅x=1/2时,x(1-x)的最大值为1/4}=1/(1-t)+4/t {令t=sin²θ}=(4-3t)/[t(1-t)] {令m=4-3t,则t=(4-m)/3}=9m/[(4-m)(m-1)]=9/[-m-4/m+5] {均值不等含兆式m+4/m>=4,m=2取等号}〉=9/[4+5]=1 (sin²θ=2/3时取等号)