25. 数列{an}\{a_{n}\}{an}中,a1=3a_{1}=3a1=3,a2=6a_{2}=6a2=6,an+2=an+1−ana_{n+2}=a_{n+1}-a_{n}an+2=an+1−an,那么a6=(a_{6}=(a6=( )))