tan45°=1
tan(45°+15°)=(tan45°+tan15°)/(1-tan45°tan15°)
=(1+tan15°)/(1-tan15°)
所以原式=1/tan60°=根号3/3
tan120=tan(83+37)=-√3
(tan83+tan37)/(1-tan83tan37)=-√3
tan83+tan37=-√3+√3tan83tan37
所以原式=-√3
tan83°+tan37°- √3tan83°tan37°
=tan(83°+37°)*(1-tan83°tan37°)- √3tan83°tan37°
=tan(120°)*(1-tan83°tan37°)- √3tan83°tan37°
=-√3