tanA+tanB+tanC
=sinA/cosA+sinB/cosB+sinC/cosC
=(sinAcosB+sinBcosA)/(cosAcosB)+sinC/cosC
=sin(A+B)/(cosAcosB)+sinC/cosC
=sinC/(cosAcosB)+sinC/cosC
=sinC[cosC+cosAcosB]/(cosAcosBcosC)
=sinC[cosAcosB-cos(A+B)]/(cosAcosBcosC)
=sinC[cosAcosB-cosAcosB+sinAsinB]/(cosAcosBcosC)
=[sinCsinAsinB]/(cosAcosBcosC)
=tanA·tanB·tanC.
证毕
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