M=1/2 + ( 1/2)^2+ ( 1/2 )^3 + .......+ (1/2 )^100
2M=2.(1/2 + ( 1/2)^2+ ( 1/2 )^3 + .......+ (1/2 )^99)
2M=1+1/2 + ( 1/2)^2+ ( 1/2 )^3 + .......+ (1/2 )^99
2M-M=1+1/2 + ( 1/2)^2+ ( 1/2 )^3 + .......+ (1/2 )^99-
1/2 + ( 1/2)^2+ ( 1/2 )^3 + .......+ (1/2 )^100
M=1-(1/2)^100