Because {tn} is a geometric series, t1t (n+2) = t2t (n+1) =1... = tit (n+3-i) = (t12) q (n

①× ②，TN ^ 2 = 10 * 10 * 10 *...* 10，* * 2 (n+2) 10。

= 10^2(n+2)

Tn= 10^(n+2)

an=lgTn=lg 10^(n+2)=n+2