根据公式1/f=1/u+1/V可得,1/V=1/f-1/u
所以,V=uf/(u-f)
因为,u＞2f,所以可以用特殊值法来验证
当u=2.5f 时,V=uf/(u-f)=2.5f×f/(2.5f-f)=5f/3
当u=3f 时,V=uf/(u-f)=3f×f/(3f-f)=3f/2
当u=3.5f 时,V=uf/(u-f)=3.5f×f/(3.5f-f)=7f/5
当u=4f 时,V=uf/(u-f)=4f×f/(4f-f)=4f/3
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当u=nf 时(n＞2),V=uf/(u-f)=nf×f/(nf-f)=nf/(n-1)
因为,n/(n-1)=1+1/(n-1),而当n＞2时,1＜1+1/(n-1)＜2
所以,f＜V＜2f