设向量a的坐标为 (x,y,z),则
cosA=x/根号(x^2+y^2+z^2),cosB=y/根号(x^2+y^2+z^2),cosC=z/根号(x^2+y^2+z^2),因此有 (cosA)^2+(cosB)^2+(cosC)^2=1,所以 (sinA)^2+(sinB)^2+(sinC)^2=2.
由柯西不等式：
[1/(sinA)^2+1/(sinB)^2+1/(sinC)^2][(sinA)^2+(sinB)^2+(sinC)^2]>=(1+1+1)^2=9
而 (sinA)^2+(sinB)^2+(sinC)^2=2,所以 1/(sinA)^2+1/(sinB)^2+1/(sinC)^2>=9/2,即所求最小值为 9/2.