解析:选C 因为y=sin+sin=sin 2xcos +cos 2xsin +sin 2xcos-cos 2xsin =sin 2x,所以所求函数的最小值为-.
6.化简sin 50°cos 38°+cos 50°cos 128°的结果为________.
解析:sin 50°cos 38°+cos 50°cos 128°=sin 50°cos 38°+cos 50°(-sin 38°)=sin 50°cos 38°-cos 50°sin 38°=sin(50°-38°)=sin 12°.
答案:sin 12°
7.已知<β<,sin β=,则sin=________.
解析:∵<β<,sin β=,
∴cos β=,∴sin=sin β·cos +cos β·sin =×+×=+=.
答案:
8.已知cos=sin,则tan α=________.
解析:cos=cos αcos-sin αsin =cos α-sin α,sin=sin αcos -cos αsin=sin α-cos α,∴sin α=cos α,故tan α=1.
答案:1
9.已知cos α=(α为第一象限角),求cos,sin的值.
解:∵cos α=,且α为第一象限角,
∴sin α== =.
∴cos=cos cos α-sin sin α
=×-×=.
同理可求sin=.
10.化简下列各式:
(1)sin+2sin-cos;