求cosπ/9*cos2π/9*cos3π/9*cos4π/9
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求cosπ/9*cos2π/9*cos3π/9*cos4π/9
cosπ/9*cos2π/9*cos3π/9*cos4π/9
=(cosπ/9*cos2π/9*cos3π/9*cos4π/9)*8(sinπ/9)/8(sinπ/9)
∵2cosπ/9*sinπ/9=sin2π/9
∴原式=(4sin2π/9*cos2π/9*cos3π/9*cos4π/9)/8(sinπ/9)
∵2sin2π/9*cos2π/9=sin4π/9
∴原式=(2sin4π/9*cos3π/9*cos4π/9)/8(sinπ/9)
=(cos3π/9*sin8π/9)/8(sinπ/9)
∵ sinπ/9=sin8π/9……诱导公式
∴原式=(cosπ/3*sin8π/9)/8(sinπ/9)
=(cosπ/3)/8
=1/16
=(cosπ/9*cos2π/9*cos3π/9*cos4π/9)*8(sinπ/9)/8(sinπ/9)
∵2cosπ/9*sinπ/9=sin2π/9
∴原式=(4sin2π/9*cos2π/9*cos3π/9*cos4π/9)/8(sinπ/9)
∵2sin2π/9*cos2π/9=sin4π/9
∴原式=(2sin4π/9*cos3π/9*cos4π/9)/8(sinπ/9)
=(cos3π/9*sin8π/9)/8(sinπ/9)
∵ sinπ/9=sin8π/9……诱导公式
∴原式=(cosπ/3*sin8π/9)/8(sinπ/9)
=(cosπ/3)/8
=1/16
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