α∈(π,5π/4)则α-π/4∈(3π/4,π)
∴sin(α-π/4)>0
sin(α-π/4)=3/5
tan(α-π/4)=-3/4
即(tanα-tanπ/4)/(1+tanαtanπ/4)=(tanα-1)/(1+tanα)=-3/4
tanα=1/7
α∈(π,5π/4)则α-π/4∈(3π/4,π)
∴sin(α-π/4)>0
sin(α-π/4)=3/5
tan(α-π/4)=-3/4
即(tanα-tanπ/4)/(1+tanαtanπ/4)=(tanα-1)/(1+tanα)=-3/4
tanα=1/7