成年未成熟 4星
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[1+cos(π/5)][1+cos(3π/5)] =1+cos(π/5)+cos(3π/5)+cos(π/5)cos(3π/5) =1+2cos(2π/5)·cos(π/5)-cos(π/5)cos(2π/5)【因为cos(3π/5)=-cos(2π/5)】 =1+cos(π/5)cos(2π/5) =1+[2sin(π/5)cos(π/5)cos(2π/5)]/[2sin(π/5)] =1+[sin(2π/5)cos(2π/5)]/[2sin(π/5)] =1+[2sin(2π/5)cos(2π/5)]/[4sin(π/5)] =1+[sin(4π/5)]/[4sin(π/5)] =1+(1/4) =5/4
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