cos5分之π×cos5分之2π
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cos(π/5)cos(2π/5)
=[4sin(π/5)cos(π/5)cos(2π/5)]/[4sin(π/5]
=[2×2sin(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)]/[sin(π/5)]
=1/4
=[4sin(π/5)cos(π/5)cos(2π/5)]/[4sin(π/5]
=[2×2sin(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)]/[sin(π/5)]
=1/4
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