Question

得出an/3^2-5/3=8/9+26/27+.(3^n-1)/3^n,等式右边的怎么化简

Answer 1

8/9+26/27+````+(3^n-1)/3^n=1-1/9+````+1-1/27+1-1/3^n

=n-1-(1/9+1/27+1/3^n)

=n-1-1/9*(1-1/3^n)/(1-1/3)

=n-1-(1-1/3^n)/6