12)cosα /(1+sinα)+tgα=cosα /(1+sinα)+sinα/cosα =
=(cos²α+sinα+sin²α)/[(1+sinα)·cosα] =(1+sinα)/[(1+sinα)·cosα] =1/cosα
3) ctgα=-2, π/2<α<π<br>
α=arcctg(-2)=π-arcctg(2)
17)sinα=-2/√29, 3π/2<α<2π<br>α=arcsin(-2/√29)=-arcsin(2/√29)
2)1-(sinαcosα)/ctgα=1-(sinαcosα)/(cosα/sinα)=1-sin²α cosα/cosα=
=1-sin²α=cos²α