作业帮cos(π÷5）＋cos(2π÷5)等于多少?不要用计算器!
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cos(π÷5）＋cos(2π÷5)等于多少?不要用计算器!
cos(π÷5）＋cos(2π÷5)等于多少?不要用计算器!
cos(π÷5）＋cos(2π÷5)等于多少?不要用计算器!
cos(π/5)-cos(2π/5)
=cos(π/5)+cos(3π/5)
=cos(2π/5-π/5)+cos(2π/5+π/5)
=cos(2π/5)*cos(π/5)+sin(2π/5)*sin(π/5)+cos(2π/5)*cos(π/5)-sin(2π/5)*sin(π/5)
=2cos(2π/5)*cos(π/5)
=2sin(π/10)*cos(π/5)
=2sin(π/10)*[cos(π/10)/cos(π/10)]*cos(π/5)
=[sin(π/5)/cos(π/10)]*cos(π/5)
=[sin(π/5)*cos(π/5)]/cos(π/10)
=(1/2)sin(2π/5)/cos(π/10)
=(1/2)cos(π/10)/cos(π/10)
=1/2
cos(π÷5）＋cos(2π÷5)等于多少?不要用计算器!
计算：cos（π/5)+cos（2π/5)+cos(3π/5)+cos(4π/5)
求值:cos(π/11)cos(2π/11)cos(3π/11)cos(4π/11)cos(5π/11)
求值：cos(π/11)-cos(2π/11)+cos(3π/11)-cos(4π/11)+cos(5π/11)
cos(π/11)cos(2π/11)cos(3π/11)cos(4π/11)cos(5π/11)
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