以2为底cosπ/9的对数+以2为底cos2π/9的对数+以2为底cos4π/9的对数=？
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原式=log2(cosπ/9cos2π/9cos4π/9)
=log2[(2sinπ/9cosπ/9cos2π/9cos4π/9)/2sinπ/9]
=log2[(sin2π/9cos2π/9cos4π/9)/2sinπ/9]
=log2[(2sin2π/9cos2π/9cos4π/9)/4sinπ/9]
=log2[(sin4π/9cos4π/9)/4sinπ/9]
=log2(sin8π/9/8sinπ/9)
=log2(sinπ/9/8sinπ/9)
=log2(1/8)
=-3
=log2[(2sinπ/9cosπ/9cos2π/9cos4π/9)/2sinπ/9]
=log2[(sin2π/9cos2π/9cos4π/9)/2sinπ/9]
=log2[(2sin2π/9cos2π/9cos4π/9)/4sinπ/9]
=log2[(sin4π/9cos4π/9)/4sinπ/9]
=log2(sin8π/9/8sinπ/9)
=log2(sinπ/9/8sinπ/9)
=log2(1/8)
=-3
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