tan30=2tan15/(1+tan²15)
2tan15/(1+tan²15)=√3/3
tan15/(1+tan²15)=√3/6
tan15+1/tan15=(tan15²+1)/tan15=6/√3=2√3
2、原式=[sinacos(a+b)+cosasin(a+b)]/sina-2cos(a+b)
=[sinacos(a+b)+cosasin(a+b)-2sinacos(a+b)]/sina
=[cosasin(a+b)-sinacos(a+b)]/sina
=sin(a+b-a)/sina
=sinb/sina
3、sinθ／2<0
cos²(θ／2)=(1+x)/x
sin²(θ／2)=1-(1+x)/x=-1/x
sinθ／2=-√(-1/x)
sinθ=2sin(θ/2)cos(θ/2)=2*[-√(-1/x)]*[√(1＋x)/x]=2[√(1+x)]/x