通分即可1/1+sinα+1/1-sinα=(1-sinα)/[(1+sinα)(1-sinα)]+(1+sinα)/[(1-sinα)(1+cosα)]=(1-sinα)/(1-sin²α)+(1+sinα)/(1-sin²α)=2/(1-sin²α)=2/cos²α=2(sin²α+cos²α)...