70°+50°=120°,
tan70°+tan50°-根号3tan70°tan50°=tan120°(1-tan70°tan50)-根号3tan70°tan50
=-根号3(1-tan70°tan50)-根号3tan70°tan50
=-根号3+根号3tan70°tan50-根号3tan70°tan50
=-根号3
70°+50°=120°,
tan70°+tan50°-根号3tan70°tan50°=tan120°(1-tan70°tan50)-根号3tan70°tan50
=-根号3(1-tan70°tan50)-根号3tan70°tan50
=-根号3+根号3tan70°tan50-根号3tan70°tan50
=-根号3