(cotα-1)/(2cotα+1)=1cotα-1=2cotα+1cotα=-2tanα=-1/2所以(cos2α)/(1+sin2α)=(1-tan²α)/(1+tan²α)】/(1+2tanα/(1+tan²α))=(1-tan²α)/(1+tan²α+2tanα)=(1-tanα)/(1+tanα)=...