# Prove the equation without the calculator

• April 21st 2009, 04:00 AM
beq!x
Prove the equation without the calculator
Prove the equation without the calculator
$8cos20^\circ cos40^\circ cos80^\circ=1$
• April 21st 2009, 04:07 AM
bebrave
8cos20.cos40.cos80
=(8sin20.cos20.cos40.cos80)/(sin20)
=(4sin40.cos40.cos80)/(sin20)
=(2sin80.cos80)/(sin20)
=2sin160/sin20=1
• April 21st 2009, 03:15 PM
beq!x
can you tell me please which formula have you used ?
and i have a little bit trouble at the end " 2sin160/sin20=1" why?