Question

[Nguyễn Xuân Dũng]

Thực hiện phép tính :

a,  S1=2+2^2+2^3+...+2^90

b,   S2=(-2)+(-2)^2+(-2)^3+...+(-2)^100

c,   S3=2^100-2^99-2^98-2^97-...-2^2-2-1

d,   S4=1-3+5-7+...+2001-2003+2005-2007

Answer 1

[Mai Ngọc]

a,  S1=2+2^2+2^3+...+2^90

=>2S1=2^2+2^3+2^4+...+2^91

=>2S1-S1=(2^2+2^3+2^4+...+2^91)-(2+2^2+2^3+...+2^90)

=>S1=2^91-1

Answer 2

[Trần Việt Hoàng]

quá khó hiểu!