1）1/2sin15°-√3/2sin75
=cos60sin15-sin60cos15
=sin(15-60)
=sin(-45)
= -√2/2
(cos15°-sin15°)/(cos15°+sin15°) 同时除以 cos15
=（1-tan15）/（1+tan15）
=tan(45-15)
=tan30
=√3/3