（1+tan15度）/（1-tan15度）

(1+tan15)/(1-tan15)
=(tan45+tan15)/(1-tan45tan15)
=tan(45+15)
=tan60
=根号3

解：tan(A+B) = (tanA+tanB)/(1-tanAtanB)

tan45=1
(1+tan15)/(1-tan15)=tan(15+45)=tan60=√3

方法2.
(1+tan15)/(1-tan15)=(1+sin15/cos15)/(1-sin15/cos15)
=(cos15+sin15)/(cos15-sin15)
=(cos15+sin15)*(cos15-sin15)/(cos15-sin15)^2
=[(cos15)^2-(sin(15)^2]/(1-2cos15sin15)
=cos30/sin30
=cot30
=√3