证明(cotA+cotB)(cotB+cotC)(cotC+cotA)=cscAcscBcscC

(cotA+cotB)(cotB+cotC)(cotC+cotA)=(cosA/sinA+cosB/sinB)(cosB/sinB+cosC/sinC)(cosC/sinC+cosA/sinA)=(cosAsinB+sinAcosB)/sinAsinB*(cosBsinC+sinBcosC)/sinBsinC*(cosCsinA+sinCcosA)/sinCsinA=sin(A+B)/sinAsi...