若(cosα-sinα)/(cosα+sinα)=4,则tan(π/4-α)等于

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若(cosα-sinα)/(cosα+sinα)=4,则tan(π/4-α)等于
若(cosα-sinα)/(cosα+sinα)=4,则tan(π/4-α)等于

若(cosα-sinα)/(cosα+sinα)=4,则tan(π/4-α)等于
(cosα-sinα)/(cosα+sinα)=4
(cosα-sinα)=4(cosα+sinα)
-5sinα=3cosα
tanα=-3/5
tan(π/4-α)=(tanπ/4-tanα)/(1-tanπ/4tanα)
=(1-tanα)/(1+tanα)
=(1-(-3/5))/(1+(-3/5))
=(8/5)/(2/5)
=4