利用公式sec^2x=1+ tan^2x,csc^2x=1+ cot^2x,sin^2x+cos^2x=1
f(x)=sin^2x+csc^2x+cos^2x+sec^2x+tan^2x+7cot^2x
= sin^2x+cos^2x+tan^2x+7cot^2x+sec^2x+csc^2x
=1+ tan^2x+7cot^2x+1+ tan^2x+1+ cot^2x
=3+2 tan^2x+8cot^2x
=3+2 tan^2x+8/ tan^2x
設(shè)tan^2x=t,x∈(0,π/4]
則t∈(0,1].
2 tan^2x+8/ tan^2x=2t+8/t,該函數(shù)在(0,2]上遞減,
所以2t+8/t的最小值是2+8=10.
∴函數(shù)f（x）的最小值是13.