（cos15°sin9°+sin6°)/（sin15°sin9°-cos6°）
=[cos15sin9+sin(15-9)]/[sin15sin9-(cos15-9)]
=(cos15sin9+sin15cos9-cos15sin9)/(sin15sin9-cos15cos9-sin15sin9)
=(sin15cos9)/(-cos15cos9)
=-tan15
=-sin30/(1+cos30)=根号3 -2.