(1) an +1- an =3·24nー1 当 n ≥2时, an - an -1=3.22( n -1)-1 Qn -1- an -2=3.22( n -2)-1 a2-Q1=3.22-1以上各项相加得 an -a1=( an - an -1) +( an -1- an -2)+.…+(a2-a1) an -a1=3.22n-8+3.22n-5+.…+3·22-1 Qn - aj =3.2(1-4nー1) 1一4 an -a1=2·4”-1-2 an =2.4”-1-2+a1=2·4”-1当 n =1时,a1=2也符合该式 故数列 an }的通项公式为 an =2×4-1