tan40-4sin40
sin40
= ----- -4sin40
cos40
sin40-4sin40cos40
= -------------------
cos40
sin40-2sin80
= --------------
cos40
sin40-sin80-sin80
= ------------------
cos40
2cos60sin(-20)-sin80
= ---------------------
cos40
sin80+sin20
= － -----------
cos40
2sin50cos30
= － -----------
cos40
= 负根号3