tanx=2
sinx/cosx=2
sinx=2cosx
由(sinx)^2+(cosx)^2=1
4(cosx)^2+(cosx)^2=1
(cosx)^2=1/5
cosx=±√5/5
sinx+2cosx
=2cosx+2cosx
=4cosx
=±4√5/5
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