# Length of Curve with multivariable

• July 8th 2009, 06:49 PM
sleepiiee
Length of Curve with multivariable
I tried to evaluate the length of this curve but cannot figure out what to do with the derivative of the log(cost). The question is:

Find the length of the curve r(t) = <1, log(cost)>
t having the domain [0, pi/4]

I did the integral from 0 to pi/4 of the square root of the sum of the derivative of the components and im having trouble integrating it. Can anyone help me solve this?
• July 8th 2009, 07:08 PM
Random Variable
$\text{length} = \int^{\pi /4}_{0} ||r'(t)|| \ dt$

$r'(t) = \Big(0, \text{-}\tan t\Big)$

$length = \int^{\pi /4}_{0} \sqrt{0 + \tan^2t} \ dt$

and since $\tan t \ge 0 \ \text{for} \ 0 \le t \le \pi /4$

$= \int^{\pi /4}_{0} \tan t \ dt$

Can you integrate that?
• July 8th 2009, 07:19 PM
sleepiiee
how did you derive log(cost) to be -tant

did you just change it to ln?
• July 8th 2009, 07:21 PM
Random Variable
Quote:

Originally Posted by sleepiiee
how did you derive log(cost) to be -tant

did you just change it to ln?

I assumed it was log base e. Is it log base 10?
• July 8th 2009, 07:53 PM
sleepiiee
its blank, would that mean its base is 10 or e?
• July 8th 2009, 08:02 PM
Random Variable
Quote:

Originally Posted by sleepiiee
its blank, would that mean its base is 10 or e?

most likely base e
• July 8th 2009, 09:20 PM
sleepiiee
i got ln|secx + tanx| evaluated at 0 and pi/4