2cos9/13πcosπ/13+cos5/13π+cos3/13π的解法?
2cos9/13πcosπ/13+cos5/13π+cos3/13π的解法?
我有一个儿子,作为母亲的我无力解决,
我有一个儿子,作为母亲的我无力解决,
赞 alonediary 幼苗
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2cos9π/13cosπ/13+cos5π/13+cos3π/13(和差化积
)
=2cos9π/13cosπ/13+2 cos[(5π/13+3π/13)/2] cos[(5π/13-3π/13)/2]
=2cosπ/13(cos9π/13+cos4π/13)
=2cosπ/13[cos(π-4π/13)+cos4π/13]
=2cosπ/13[-cos4π/13)+cos4π/13]
=2cosπ/13*0
=0
)
=2cos9π/13cosπ/13+2 cos[(5π/13+3π/13)/2] cos[(5π/13-3π/13)/2]
=2cosπ/13(cos9π/13+cos4π/13)
=2cosπ/13[cos(π-4π/13)+cos4π/13]
=2cosπ/13[-cos4π/13)+cos4π/13]
=2cosπ/13*0
=0
1年前
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