化简（1-sina-cosa)÷(1+cosa-sina)   （a≠2kπ+π/2,k∈Z) 等于

(1-sina-cosa)/(1+cosa-sina)
=[1-(sina+cosa)]/[1-(sina-cosa)]
=[1-(sina+coaa)][1-(sina+cosa)]/[1^2-sina-cosa-sina+cosa+sin^2a-cos^2a]
=[1^2-2(sina+cosa)+sina^2+2sinacosa+cos^2a]/[1-2sina+sin^2a-cos^2a]
=[2-2(sina+cosa)+2sinacosa]/[2sin^2a-2sina]
=[1-sina-cosa+sinacosa]/[sin^2a-sina]
=[(1-sina)(1-cosa)/sina(sina-1)
=(cosa-1)/sina
=cota-csca

（1-sina-cosa)÷(1+cosa-sina)
=(1-sina-cosa)^2÷[(1-sina)^2-cos^2a]
=(2-2sina-2cosa+2sinacosa)÷(2sin^2a-2sina)
=(1-sina)(1-cosa)/[(sina-1)*sina]
=(cosa-1)/sina.（a≠2kπ+π/2,k∈Z) .

(1-sina-cosa)/(1+cosa-sina)
=(1-2sina/2cosa/2-1+2(sina/2)^2)/(1+2(cosa/2)^2-1-2sina/2cosa/2)
=(2(sina/2)^2-2sina/2cosa/2)/(2(cosa/2)^2-2sina/2cosa/2)
=[2sina/2(sina/2-cosa/2)]/[2cosa/2(cosa/2-sina/2)]
=-(sina/2)/(cosa/2)
=-tana/2