原式=(sinA-1/sinA)(cosA-1/cosA)(tanA-1/tanA) =[(sin^2-1)/sinA][(cos^2A-1)cosA][(tan^2A-1)tanA] =(cos^2A/sinA)(sin^2A/cosA)(2/tan2A) =2sinAcosA/tan2A =sin2A/tan2A =cos2A