1.设f(θ)=,则f=________.
解析:f(θ)=
=
=
=
==cos θ-1.
∴f=cos -1=-.
答案:-
2.已知sin α+cos α=,α∈(0,π),求tan α的值.
解:由sin α+cos α=,①
得2sin αcos α=-<0,
∴α∈,
∴sin α-cos α>0.
又(sin α-cos α)2=1-2sin αcos α=,
∴sin α-cos α=.②
由①②得sin α=,cos α=-,
∴tan α=-.
3.已知sin(180°+α)=-,0°<α<90°,
求的值.