1. If S = 1+2+2+ .....................................................................................................................................................

The two subtractions are: 2s-s = 2+22+23++2n+2 (n+1)-1+2+2n,

S=2^(n+ 1)- 1 .

2. let s =1+a+a 2+...+an n, then As = A+A 2+A 3+...+A N+A (n+ 1),

The two subtractions are as-s = [a+a 2+a 3+...+an+a (n+1)]-[1+a 2+...+an],

(a-1) s = a (n+1)-1,when a≠ 1, there are: s = a (n+1)-1

Third, if 0

= 1/( 1-a)-a^(n+ 1)/( 1-a)<； 1/( 1-a)。