1 cos20*cos40*cos60*cos80
=16sin20cos20*cos40*cos60*cos80/16sin20
=sin160/(sin20*16)
=1/16
2 (1/cos50) +tan10
=1/cos50+sin10/cos10
=2sin50/sin100+sin10/cos10
=(2sin50+sin10)/cos10
=(2cos40+sin10)/cos10
=√3cos10/cos10
=√3
3 (1)sinx-cosx>0
x属于(∏/4+k∏,3∏/4+k∏)
(2)tanx>0
2cosx+1≥0
cosx≥-1/2 tanx>0
或cos≤-1/2 tanx