Area between line and curve

• May 7th 2008, 03:33 PM
zsig013
Area between line and curve
Find the area between
y = 5x^3 and the line y = 45x, for x http://highered.mcgraw-hill.com/tber.../chars/geq.gif 0

Can anybody show me how to go about doing this?
• May 7th 2008, 03:48 PM
icemanfan
Quote:

Originally Posted by zsig013
Find the area between
y = 5x^3 and the line y = 45x, for x http://highered.mcgraw-hill.com/tber.../chars/geq.gif 0

Can anybody show me how to go about doing this?

First you need to figure out where the curves intersect. So you have the equation $5x^3 = 45x$ or $5x^3 - 45x = 0$. Simplifying, expanding and solving for x:

$5(x^3 - 9x) = 0$

$x^3 - 9x = 0$

$x(x^2 - 9) = 0$

$x(x-3)(x+3) = 0$

So the curve has three roots. But the problem asks us only to consider values of x greater than or equal to 0, so we have the points of intersection at x = 0 and x = 3.

The next question is, which of the two curves is above the other? A quick test reveals $f(1) = -40$ where $f(x) = 5x^3 - 45x$ so it must be that $45x$ is above $5x^3$. Hence, you want to use $g(x) = 45x - 5x^3$.

So to compute the area between the curves we integrate

$\int_{0}^{3} 45x - 5x^3 dx$.
• May 7th 2008, 03:56 PM
zsig013
Awesome, thanks!