Question

数列求和 1+（-1/2）+.+（-1/2）的n-2次方 为什么 = 1-(-1/2)den-1/1--1/2 右面次方

Answer 1

sn=1+(-1/2)+(-1/2)^2……+(-1/2)^(n-3)+(-1/2)^(n-2)

两边同乘以(-1/2)

(-1/2)sn=(-1/2)+(-1/2)^2+(-1/2)^3……+(-1/2)^(n-2)+(-1/2)^(n-1)

两式相减：

(3/2)sn=1-(-1/2)^(n-1)

sn=2/3-(2/3)(-1/2)^(n-1)