# Thread: Prove the equation without the calculator

1. ## Prove the equation without the calculator

Prove the equation without the calculator
$8cos20^\circ cos40^\circ cos80^\circ=1$

2. 8cos20.cos40.cos80
=(8sin20.cos20.cos40.cos80)/(sin20)
=(4sin40.cos40.cos80)/(sin20)
=(2sin80.cos80)/(sin20)
=2sin160/sin20=1

3. can you tell me please which formula have you used ?
and i have a little bit trouble at the end " 2sin160/sin20=1" why?