• Jul 17th 2006, 12:35 AM
a) Explain why the graph of the function f(x) = (x-3)(e^-x/4 - 1) lies above the x- axis for 0<x<3 and below x-axis for x>3

b) Use this fact to find the area enclosed by the graph and the x-axis between x = 0 and x = 6, giving your answer to 4 d.p
• Jul 17th 2006, 10:57 AM
ThePerfectHacker
Quote:

Originally Posted by fair_lady0072002
a) Explain why the graph of the function f(x) = (x-3)(e^-x/4 - 1) lies above the x- axis for 0<x<3 and below x-axis for x>3

You have,
$f(x)=\frac{x-3}{e^{-x/4}-1}$
You want to find where it is below the x-axis thus, you want to find,
$f(x)<0$
Thus,
$\frac{x-3}{e^{-x/4}-1}<0$
You are dividing two fractions.
They are negative when they have opposite signs,
$x-3<0 \mbox{ and }e^{-x/4}-1 >0$ (1)
Or,
$x-3>0 \mbox{ and }e^{-x/4}-1 <0$ (2)
---
Now solve both inequalities.
(1) The first part is easy $x-3<0$ thus, $x<3$
Now, the second part,
$e^{-x/4}-1>0$
Thus,
$e^{-x/4}>1$
Okay, you need to know something about exponential curves.
An exponential curve $a^x$ is increasing and intersectest y-axis at $x=0$ An exponential curve $a^{-x}$ is decreasing and intersectest y-axis at $x=0$.

This exponent $e^{-x/4}$ is of the type $a^{-x}$ So it is decreasing and intersects the y-axis (which is one) at $x=0$ Thus, when $x>0$ The function (since decreasing) is always less than one.

You now have for the first part $x<3 \mbox{ and }x>0$, a more fancy way to say this is $0.
---
For the second part (2), following same ideas you arrive at,
$x>3 \mbox{ and } x<0$, but this set is empty (basically impossible, right?) Thus, you disregard this case.
• Jul 17th 2006, 11:10 AM
ThePerfectHacker
Quote:

Originally Posted by fair_lady0072002
b) Use this fact to find the area enclosed by the graph and the x-axis between x = 0 and x = 6, giving your answer to 4 d.p

Whoa!
Remember on the interval $[0,6]$ the function is not countinous (discountinous at $x=0$). Since that point is a vertical asymptote, This is a Type II Improper Integral.
Furthermore, It is negative on $(0,3)$ and positive on $(3,6]$
Therefore you need to do the integrals
$\int_{0^+}^3 \frac{x-3}{e^{-x/4}-1} dx-\int_3^6 \frac{x-3}{e^{-x/4}-1}dx$
The reason why a negative because the function is negative there.
And also remember that,
$\int_{0^+}_3 \frac{x-3}{e^{-x/4}-1} dx=\lim_{N\to 0^+}\int_{N}^3 \frac{x-3}{e^{-x/4}-1} dx$
• Jul 17th 2006, 03:20 PM
galactus
Quote:

Originally Posted by fair_lady0072002
a) Explain why the graph of the function f(x) = (x-3)(e^-x/4 - 1) lies above the x- axis for 0<x<3 and below x-axis for x>3

b) Use this fact to find the area enclosed by the graph and the x-axis between x = 0 and x = 6, giving your answer to 4 d.p

Isn't that multiplication instead of division?.
• Jul 17th 2006, 04:06 PM
ThePerfectHacker
Quote:

Originally Posted by galactus
Isn't that multiplication instead of division?.

Why do I always do that?

(Does that mean that this would tear a hole in the spacetime countinuum)