Prove the equation without the calculator

$\displaystyle 8cos20^\circ cos40^\circ cos80^\circ=1$

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- Apr 21st 2009, 03:00 AMbeq!xProve the equation without the calculator
**Prove the equation without the calculator**

$\displaystyle 8cos20^\circ cos40^\circ cos80^\circ=1$ - Apr 21st 2009, 03:07 AMbebrave
8cos20.cos40.cos80

=(8sin20.cos20.cos40.cos80)/(sin20)

=(4sin40.cos40.cos80)/(sin20)

=(2sin80.cos80)/(sin20)

=2sin160/sin20=1 - Apr 21st 2009, 02:15 PMbeq!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?