（1）cos 17π/12 =cos(π+5π/12)=-cos5π/12=-sinπ/12=-sin15度=-sin(45-30)=-根号2/4+根号6/4
（2）sin（-61π/12）=sin(-6π+11π/12)=sin11π/12=sin(π-π/12)=sinπ/12=-上式