tan(α-5/4π)
=tan[(α-π/4)-π]
=tan(α-π/4)
=(tanα-tanπ/4)/(1+tanαtanπ/4)
=(tanα-1)/(1+tanα)
于是，(tanα-1)/(1+tanα)=1/5
5tanα-5=1+tanα
4tanα=6
tanα=3/2
tan(α-5π/4)=1/5
tan(-π+α-π/4)=1/5
tan(α-π/4)=1/5
(tanα-tanπ/4)/(1+tanαtanπ/4)=1/5
(tanα-1)/(1+tanα)=1/5
1+tanα=5tanα-5
4tanα=6
tanα=3/2
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