# Homework Help: Exponential function sum problem

1. Feb 2, 2012

### rustyjoker

1. The problem statement, all variables and given/known data
I need to prove that
(1+$x_{1})$·...·(1+$x_{n}$)≥(1-$Ʃ^{n}_{i=1}x_{i}^2$)$e^{Ʃ^{n}_{i=1}x_{i}}$
with all 0≤$x_{i}$≤1

(1+$x_{1}$)·...·(1+$x_{n}$)≤$e^{Ʃ^{n}_{i=1}x_{i}}$
with all 0≤$x_{i}$≤1

and (1-$x_{1}$)·...·(1-$x_{i}$)≥1-Ʃ$^{n}_{i=1}x_{i}$ with all 0≤$x_{i}$≤1 ,

but can't figure out what to do with the main problem :D

2. Feb 2, 2012

### sunjin09

Take logarithm on both sides, i.e.,
Ʃ log(1+x_i)≥log(1-Ʃ x_i^2)+Ʃ x_i
then realize that x(1-x)≤log(1+x)≤x for 0<x<1, substitute in and you'll see

3. Feb 7, 2012

### rustyjoker

you cant take logarithm because you can't know if 1-sum(x_i)^2 is negative or not.