tan10°+tan50°+tan120°tan10°•tan50°的值应是(  ) A.-1 B.1 C.-3 D.3

问题描述:

tan10°+tan50°+tan120°
tan10°•tan50°
的值应是(  )
A. -1
B. 1
C. -
3

D.
3

tan10°+tan50°+tan120°
tan10°•tan50°
=
tan10°+tan50°
tan10°•tan50°
+
tan120°
tan10°•tan50°
 
=
tan60°(1−tan10°tan50°)
tan10°•tan50°
-
tan60°
tan10°•tan50°
 
=
3
tan10°tan50°
-
3
-
3
tan10°tan50°
=-
3

故选C.