tan（π/4+α）=k 求cos2α

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tan（π/4+α）=k求cos2αtan（π/4+α）=k求cos2αtan（π/4+α）=k求cos2αcos2α=sin(Pi/2+2α)=sin(2(Pi/4+α))=2tan(π/4+α))

tan（π/4+α）=k 求cos2α
tan（π/4+α）=k 求cos2α

tan（π/4+α）=k 求cos2α
cos2α
=sin(Pi/2+2α)
=sin( 2(Pi/4+α) )
=2tan(π/4+α))/(1+tan^2(π/4+α))
=2k/(1+k^2)