A.cos(A+B)=cos C B.sin(A+B)=-sin C
C.cos=sin B D.sin=cos
解析:选D ∵A+B+C=π,∴A+B=π-C,
∴cos(A+B)=-cos C,sin(A+B)=sin C,故A,B错.
∵A+C=π-B,∴=,
∴cos=cos=sin,故C错.
∵B+C=π-A,∴sin=sin=cos,故D正确.
6.sin 95°+cos 175°的值为________.
解析:sin 95°+cos 175°=sin(90°+5°)+cos(180°-5°)
=cos 5°-cos 5°=0.
答案:0
7.若sin=,则cos2θ-sin2θ=________.
解析:sin=cos θ=,从而sin2θ=1-cos2θ=,所以cos2θ-sin2θ=-.
答案:-
8.化简:sin(-α-7π)·cos=________.
解析:原式=-sin(7π+α)·cos
=-sin(π+α)·
=sin α·(-sin α)
=-sin2α.
答案:-sin2α
9.已知sin(π+α)=-.
求:(1)cos;
(2)sin.