(2017北大特优)求$9\tan 10^\circ+2\tan 20^\circ+4\tan 40^\circ-\tan 80^\circ=$_____
A．$0$
B.$\dfrac{\sqrt 3}3$
C．$1$
D．$\sqrt 3$
提示:$\cot x-\tan x=2\cot(2x)$得答案0
(2017北大特优)求$9\tan 10^\circ+2\tan 20^\circ+4\tan 40^\circ-\tan 80^\circ=$_____
A．$0$
B.$\dfrac{\sqrt 3}3$
C．$1$
D．$\sqrt 3$
提示:$\cot x-\tan x=2\cot(2x)$得答案0