9x^2+4y^2=1
x^2/(1/9)+y^2/(1/4)=1 焦点在y轴上
a²=1/4,b²=1/9
c²=5/36
c=(√5)/6
=>椭圆9x^2+4y^2=1的焦点坐标(0,√5)/6),(0,-(√5)/6)