8. 已知等比数列{an}\{a_{n}\}{an}的前nnn项和为SnS_{n}Sn,2S3=a4−a12S_{3}=a_{4}-a_{1}2S3=a4−a1,且a2+a4=15a_{2}+a_{4}=15a2+a4=15,则a3+a5=(a_{3}+a_{5}=(a3+a5=( )))