10.已知SnS_{n}Sn为数列{an}\{a_{n}\}{an}的前nnn项和,且Sn=2an(n⩾2)S_{n}=2a_{n}(n\geqslant 2)Sn=2an(n⩾2),若S5=8S_{5}=8S5=8,则a6=a_{6}=a6=[ 8 ].