看不懂?可以清楚点吗
追答哪个地方没看懂啊
a1+2a2+2²a3+......+2^n-1乘an=n² (1)
a1+2a2+2²a3+......+2^n-1乘an+2^n乘an+1=(n+1)² (2)
(2)-(1) 错位相减法:得
(n+1)²-n²=2^n乘an+1
2n+1=2^n乘an+1
an+1=(2n+1)/2^n
an=(2n11)/2^(n-1)
第二问呢?
追答an=(2n-1)/2^(n-1)
sn=1+3/2+5/2^2+..+(2n-1)/2^(n-1)
1/2sn=1/2+3/2^2+5/2^3+..+(2n-1)/2^n
错位相减法
1/2sn=1+(3/2-1/2)+(5/2^2-3/2^2)+...+[(2n-1)/2^(n-1)-(2n-3)/2^(n-1)-(2n-1)/2^n
1/2sn=1+1+1/2+...+1/2^(n-2)-(2n-1)/2^(n-1)
sn=2+2+1+...+1/2^(n-1)-(2n-1)/2^n
sn=4-(2n-1)/2^n+1+...+1/2^(n-1)
sn=4-(2n-1)/2^n+(1-1/2)/(1-1/2^(n-1))
sn=4-(2n-1)/2^n+1/(1-1/2^n)