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KnightsPG
04-22-2002, 02:24 PM
This questions comes from the Averbach-Mehta study guide, practice exam 3, question # 32

A curve in R^3 has parametric equations x = 2(t^1.5), y = 2t + 1, z = squareroot 3 * t. Calculate the length of the curve between the points corresponding to t = 1, t = 2.

My answer follows the answer key until we get to the integral from 1 to 2 of squareroot of 9t + 7.

I use substitution: u = 9t + 7. du = 9, hence we have...

step 1: integral from 16 to 25 of squareroot u times 1/9
step 2: 2(u^1.5)/27 evaluated from 16 to 25
step 3: 250/27 - 64/27 = 186/27 = 62/9

The answer key gives an answer of 122/27. Can anyone help me out? Am I working through the steps of substitution wrong?

Thanks!

Dr T Non-Fan
04-22-2002, 02:28 PM
You forgot to multiply 64 by 2 in the second term.

KnightsPG
04-22-2002, 02:32 PM
Thanks. I'm an idiot.

Dr T Non-Fan
04-22-2002, 08:07 PM
A simple mistake. Nothing a vice grip can't fix.

The Drunken Actuary
04-22-2002, 08:48 PM
DTNF, you're my hero.

KnightsPG
04-23-2002, 09:33 AM
DTNF: you're my hero too. I'd even agree with you on the separate exam threads if I didn't have some non-exam 1 takers like you helping me get through this exam.

ASA_Woman
04-23-2002, 09:41 AM
Hmmm. KnightsPG makes a good point.

Dr T Non-Fan
04-23-2002, 02:23 PM
I'm not taking any of Courses 1 through 4.

Hero? I'm just a mild-mannered guy. Don't listen to TDA. I now recommend Everclear with his Coke.

Dr T Non-Fan
04-23-2002, 04:10 PM
Actually, Knight makes a better point for contraction than for keeping things as they are today.