4cos20cos40cos60cos80=1/4
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(2×2cos20.sin20.cos40.cos60.cos80)/sin20
(2×sin40.cos40.cos60.cos80)/sin20
(sin80.cos80.cos60)/sin20
{2×(sin80.cos80.cos60)}/2sin20
(sin160.cos60)/2sin20
1/(2×2)
1/4
(2×sin40.cos40.cos60.cos80)/sin20
(sin80.cos80.cos60)/sin20
{2×(sin80.cos80.cos60)}/2sin20
(sin160.cos60)/2sin20
1/(2×2)
1/4
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