已知tana=-2,求sina+cosa/sina-cosa,sinacosa

tana=sina/cosa=-2,
sina=-2cosa
(1)(sina+cosa)/(sina-cosa)
=(-2cosa+cosa)/(-2cosa-cosa)
=-cosa/-3cosa
=1/3
(2)tana=-2,sina=-2cosa
tan2a=sin2a/cos2a
=2tana/(1-tan^a)
=2*(-2)/(1-4)
=4/3
即
sin2a=4/3cos2a
=(4/3)(2cos^a-1)
=(8/3)cos^a-4/3
sinacosa=-2cos^a
=1/2sin2a
=1/2*[(8/3)cos^a-4/3]=(4/3)cos^a-2/3
-2cos^a=(4/3)cos^a-2/3
即(-10/3)cos^a=-2/3
cos ^a=1/5
故sinacosa=-2cos^a=-2/5