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log2cosπ/5+log2cos2π/5
=log2(cosπ/5*cos2π/5)
=log2[(2sinπ/5*cosπ/5*cos2π/5)/2sinπ/5]
=log2[(sin2π/5*cos2π/5)/2sinπ/5]
=log2[(sin4π/5)/2*2sinπ/5]
=log2(1/4)
=-2
=log2(cosπ/5*cos2π/5)
=log2[(2sinπ/5*cosπ/5*cos2π/5)/2sinπ/5]
=log2[(sin2π/5*cos2π/5)/2sinπ/5]
=log2[(sin4π/5)/2*2sinπ/5]
=log2(1/4)
=-2
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