Giải thích các bước giải:

Ta có:

$\sin50^o\cos(-30^o)\cdot\cot120^o\sin(-120^o)\cdot\tan(\dfrac65\pi)\cdot\tan(\dfrac{-\pi}7)$

$=\sin50^o\cdot \cos30^o\cdot\cot120^o(-\sin120^o)\cdot\tan(\dfrac65\pi)\cdot(-\tan\dfrac{\pi}7)$

$=\sin50^o\cos30^o\cot120^o\sin120^o\tan(\dfrac65\pi)\tan\dfrac{\pi}7$

Ta có:

$0^o<50^o<180^o\to \sin50^o>0$

$0<30^o<90^o\to \cos30^o>0$

 $90^o<120^o<180^o\to \cot120^o<0$

$0^o<120^o<180^o\to \sin120^o>0$

$\pi<\dfrac65\pi<\dfrac32\pi\to \tan(\dfrac65\pi)>0$

$0<\dfrac{\pi}7<\dfrac12\pi\to\tan\dfrac{\pi}7>0$

$\to \sin50^o\cos30^o\cot120^o\sin120^o\tan(\dfrac65\pi)\tan\dfrac{\pi}7<0$