- 数学(sin15cos5
- (sin15c5-sin20)/(cos15cos5-cos20)
- sin20=sin(15+5)=sin15cos5+cos15sin5
cos20=cos(15+5)=cos5cos15-sin5sin15,
所以
(sin15cos5-sin20)/(cos15cos5-cos20)
=(-cos15sin5)/(sin5sin15)=-cot15=-1/(2-根号{3})=-2-根号{3}
tan15=tan(60-45)=(根号{3}-1)/(1+根号{3})=2-根号{3}