Question

an=1+2+2²+2³+.......+2的n-1次方等于多少？

an=1+2+2²+2³+.......+2的n-1次方等于多少？数列问题

Answer 1

an=1+2+2²+2³+.......+2^(n-1)=1*(1-2^n)/(1-2)=-(1-2^n)=(2^n)-1
an=(2^n)-1

Answer 2

an = 1+2^1+2^2+...+2^(n-1) (1)
2an = 2^1+2^2+2^3+...+2^n (2)

(2)-(1)
an = 2^n - 1