又sin30=sin(15+15)=2sin15cos15=0.5
又(sin15)^2+(cos15)^2=1
即(sin15+cos15)^2=1.5
(sin15-cos15)^2=0.5
即sin15+cos15=根號下1.5=根號6/4
cos15-sin15=根號下0.5=根號2/2
那麽tan15=sin15/cos15=(根號2/2-根號6/4)/(根號2/2+根號6/4)
=(2根號2-根號6)/(2根號2+根號6)
=(14-8根號3)/2