cosπ/5cos2π/5

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cosπ/5cos2π/5cosπ/5cos2π/5cosπ/5cos2π/5cos(π/5)cos(2π/5)=4sin(π/5)cos(π/5)cos(2π/5)/[4sin(π/5)]=2sin

cosπ/5cos2π/5
cosπ/5cos2π/5

cosπ/5cos2π/5
cos(π/5)cos(2π/5)
=4sin(π/5) cos(π/5)cos(2π/5)/ [4sin(π/5)]
=2sin(2π/5) cos(2π/5)/ [4sin(π/5)]
=sin(4π/5)/ [4sin(π/5)]
=sin(π/5)/ [4sin(π/5)]
=1/4.