y=4/cos^2x+9/sin^2x=4/(1-sin^2x)+9/sin^2x=[4/(1-sin^2x)+9/sin^2x](1-sin^2x+sin^2x)=4+9+4乘以[sin^2x/(1-sin^2x)]+9乘以(1-sin^2x/sin^2x)均值不等式,y>=4+9+12=25,等号成立,sin^2x=3/5