f（x）=(sin²x-sinxcosx)/(1+cos2x)
=(sin²x-sinxcosx)/(2cos²x)
=1/2*(tan²x-tanx)
=1/2*[(tanx-1/2) ²-1/4]
因为00.
(tanx-1/2) ²-1/4≥-1/4,
∴函数最小值是-1/8.