tan120=(tan70+tan50)/(1-tan70tan50)
所以tan70+tan50=tan120(1-tan70tan50)=-√3(1-tan70tan50)
tan70+tan50-√3tan70tan50°
=-√3(1-tan70tan50)-√3tan70tan50
=-√3
tan120=(tan70+tan50)/(1-tan70tan50)
所以tan70+tan50=tan120(1-tan70tan50)=-√3(1-tan70tan50)
tan70+tan50-√3tan70tan50°
=-√3(1-tan70tan50)-√3tan70tan50
=-√3