∵A B C依次成等差数列∴2B=A+C∴3B=180°∴B=60°∴A+C=120°1/cosA +1/cosC=-√2/cosB∴(cosA+cosC)/cosAcosC=√2cos(A+C)带入A+C=120°∴(cosC+cosA)/cosCcosA=-2√2∴2cos[(A+C)/2][cos(A-C)/2]=-√2[cos(A+C)+c...