| 1 a+ 1|

=(a+ 1)^2-a=a^2+a+ 1=(a^3- 1)/(a- 1)

d3=(a+ 1)d2-a(a+ 1)=a^3+a^2+a+ 1=(a^4- 1)/(a- 1)

d3=(a+ 1)d3-ad2=a^4+a^3+a^2+a+ 1=(a^5- 1)/(a- 1)

……

Through mathematical induction, it can be proved that:

d(n)=[a^(n+ 1)- 1]/(a- 1)