得                                   

19．（1）由是等比數(shù)列且

                                                        14′

(2)由

得  accosB=b²-(a-c)²                                           9′

即accosB=a²+c²-2accosB-(a²+c²-2ac)                             11′

∴最大角為C,有                 5′

C=120°                                                      7′

18． 在中有sinA:sinB=3:5  

∴a : b=3 : 5                                                     2′

令a = 3k  (k>0)    ∴ b = 5k

∵a,b,c成等差數(shù)列    ∴ c =7k

綜上所述：