1. 程式人生 > >(二)證明數列{(1+1/n)^(n+1)}為遞減數列,{(1+1/n)^(n)}為遞增數列

(二)證明數列{(1+1/n)^(n+1)}為遞減數列,{(1+1/n)^(n)}為遞增數列

1. 利用不等式bn+1an+1>(n+1)an(ba),b>a>0證明 {(1+1n)n+1} 為遞減數列

證明:

bn+1an+1>(n+1)an(ba)
可得 bn+1>[(n+1)(ba)+a]an
b=1+1n;a=1+1n+1
可得(1+1n)n+1>[(n+1)(1n1n+1)+1+1n+1](1+1n+1)n
整理(1+1n)n+1>(1+1n+1n+1)(1+1n+1)n
由於1n1n+1>1(n+1)2
帶入可得(1+1n)n+1>(1+2n+1+1(n+1)2)(1+1n+1)n=
(1+1n+1)2(1+1n+1)n

所以(1+1n)n+1>(1+1n+1)n+2
所以數列{(1+1n)n+1}為遞減數列

2. 利用不等式bn+1an+1<(n+1)bn(ba),b>a>0證明 {(1+1n)n} 為遞增數列

證明:

bn+1an+1<(n+1)