(3) 2 tanx = 4 tanx/2/(1-tanx/2 2
tan(x/2+π/4)+tan(x/2-π/4)=(tanx/2+ 1)/( 1-tanx/2)+(tanx/2- 1)/( 1+tanx/2)
=4tanx/2/( 1-tanx/2^2);
⑸( 1-2sinαcosα)/(cosα^2-sinα^2)=(sinα^2+cosα^2-2sinαcosα)/(cosα^2-sinα^2)
=(cosα-sinα)^2/(cosα-sinα)(cosα+sinα)=cosα-sinα/cosα+sinα
=( 1-sinα/cosα)/( 1+sinα/cosα)/=( 1-tanα)/( 1+tanα);
(7)( 1-cos2θ)/( 1+cos2θ)={ 1-(2cosθ^2- 1)}/{ 1+(2cosθ^2- 1)}=(2-2cosθ^2)/2cosθ^2
=2sinθ^2/2cosθ^2=tanθ^2