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Math Help - Prove the equation without the calculator

  1. #1
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    Prove the equation without the calculator

    Prove the equation without the calculator
    8cos20^\circ cos40^\circ cos80^\circ=1
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  2. #2
    Junior Member bebrave's Avatar
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    8cos20.cos40.cos80
    =(8sin20.cos20.cos40.cos80)/(sin20)
    =(4sin40.cos40.cos80)/(sin20)
    =(2sin80.cos80)/(sin20)
    =2sin160/sin20=1
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  3. #3
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    can you tell me please which formula have you used ?
    and i have a little bit trouble at the end " 2sin160/sin20=1" why?
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