Chứng minh 1+ tan x+ tan x^2+ tan x^3= sĩn + cos x / cos^3 x .
Chứng minh 1+tanx+tan2x+tan3x=sinx+cosxcos3x.
Chứng minh 1+tanx+tan2x+tan3x=sinx+cosxcos3x.
1+tanx+tan2x+tan3x
=cos3x+sinx.cos2x+sin2x.cosx+sin3xcos3x
=cos2x.(sinx+cosx)+sin2x.(sinx+cosx)cos3x
Vậy 1+tanx+tan2x+tan3x=sinx+cosxcos3x.