①sinα=3/5,α∈（π/2,π）,
则cosα=-4/5,tanα=-3/4,
tan(β-π)=1/2,则tanβ=1/2,
则tan(α-β)=( tanα-tanβ)/(1+ tanαtanβ)=-2.
②sinα=-3/5,α是第四象限角,
则cosα=4/5,tanα=-3/4,
则tan(α-π/4)= ( tanα-tanπ/4)/(1+ tanαtanπ/4)=-7.