问题标题：
已知tana/tana-1=-1,求下列各式的值1)sina-3cosa/sina+cosa2)sina^2+sinacosa+2
问题描述：
已知tana/tana-1=-1,求下列各式的值
1)sina-3cosa/sina+cosa
2)sina^2+sinacosa+2
代应回答：
tana/(tana-1)=-1
tana=1-tana
2tana=1
tana=1/2
(sina-3cosa)/(sina+cosa)分子分母同时除以cosa
=(sina/cosa-3cosa/cosa)/(sina/cosa+cosa/cosa)
=(tana-3)/(tana+1)
=(1/2-3)/(1/2+1)
=(-5/2)/(3/2)
=-5/3
sina^2+sinacosa+2
=(sina^2+sinacosa+2)/1
=[sina^2+sinacosa+2(sina^2+cosa^2)]/(sina^2+cosa^2)
=(3sina^2+sinacosa+2cosa^2)/(sina^2+cosa^2)分子分母同时除以cosa^2
=(3sina^2/cosa^2+sinacosa/cosa^2+2cosa^2/cosa^2)/(sina^2/cosa^2+cosa^2/cosa^2)
=(3tana^2+tana+2)/(tana^2+1)
=[3*(1/2)^2+1/2+2]/[(1/2)^2+1]
=(3/4+1/4+2)/(1/4+1)
=3/(5/4)
=3*4/5
=12/5
