解由等比数列{an}的前n项和为Sn,知sm,s2m-sm,s3m-s2m,s4m-s3m成等比数列则由S4=3S8-S4=15-3=12S12-S8=12×3=36S16-S12=36×3=108故a11+a12+a13+a14=S16-S12=108.