6.已知数列{an}\{a_{n}\}{an}满足a1=2a_{1}=2a1=2,an+1=1+an1−ana_{n+1}=\frac{1+{a_n}}{1-{a_n}}an+1=1−an1+an,其前nnn项和为TnT_{n}Tn,则T15=(T_{15}=(T15=( )))