# Homework Help: Australian HSC maths extension 2 test question

1. Oct 27, 2009

### Mentallic

1. The problem statement, all variables and given/known data
This question is from the Australian HSC maths extension 2 test. Q8b)

Let n be a positive integer greater than 1.

The area of the region under the curve y=1/x from x=n-1 to x=n is between the areas of two rectangles.

Show that $$e^{-\frac{n}{n-1}}<\left(1-\frac{1}{n}\right)^n<e^{-1}$$

3. The attempt at a solution

The area under the curve is more than the smaller rectangle but less than the larger rectangle.

Thus, $$\frac{1}{n}<\int^n_{n-1}\frac{dx}{x}<\frac{1}{n-1}$$

After manipulating somewhat:

$$\frac{1}{n}<ln\left(\frac{n}{n-1}\right)<\frac{1}{n-1}$$

$$e^{\frac{1}{n}}<\frac{n}{n-1}<e^{\frac{1}{n-1}}$$

but I'm unsure how to get to the answer...

2. Oct 27, 2009

### lanedance

Re: Inequalities

try inverting everything as a next step, then consider raising everything to a power

3. Oct 27, 2009

### Mentallic

Re: Inequalities

The reciprocal... how could I miss that... thanks lanedance.