tan(－25π/12)=－tan(25π/12)=－tan(π/12)
因为tan(a/2)=sina/(1＋cosa),则tan(π/12)=sin(π/6)/[1＋cos(π/6)]=(1/2)/[1＋(√3/2)]=2－√3
则：tan(－25π/12)=√3－2