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 June 11th, 2017, 04:38 PM #1 Senior Member   Joined: Jan 2017 From: Toronto Posts: 152 Thanks: 2 parameterization and Integral One parameterization for the unit circle C is r(t) = , for 0 <= t <= pi. Set up but do not evaluate the line integral of f(x, y) = x^2 + y^2 + 3 along C, using the parameterization above. Answer $ \int_{0}^{pi} 8 dt$ But.. according to my calculation, I got 4 dt only My reasoning: x^2+y^2+3 = (sin(2t))^2 + (cos(2t))^2+3 = 1 + 3 = 4 Any idea where I got it wrong??
 June 11th, 2017, 04:51 PM #2 Global Moderator   Joined: Dec 2006 Posts: 18,235 Thanks: 1437 You got 4 dt only? With no integral limits?
June 11th, 2017, 05:05 PM   #3
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Quote:
 Originally Posted by skipjack You got 4 dt only? With no integral limits?

Here is the limit: 0 <= t <= pi

 June 11th, 2017, 06:18 PM #4 Global Moderator   Joined: Dec 2006 Posts: 18,235 Thanks: 1437 Okay. By default, a line integral is with respect to s, not t. For this problem, ds/dt = 2, so the answer given (in terms of t) is twice the answer you found. Do you know the formula by which ds/dt can be found?
June 11th, 2017, 06:20 PM   #5
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Quote:
 Originally Posted by skipjack Okay. By default, a line integral is with respect to s, not t. For this problem, ds/dt = 2, so the answer given (in terms of t) is twice the answer you found. Do you know the formula by which ds/dt can be found?
You think I may have missed putting some vital info about the question?

 June 11th, 2017, 11:12 PM #6 Global Moderator   Joined: Dec 2006 Posts: 18,235 Thanks: 1437 You should previously have been shown a comparable example of a line integral. You have to obtain $\displaystyle \int_0^\pi \!\text{f}(x,y)\frac{ds}{dt}dt$, where $\displaystyle \frac{ds}{dt} = \sqrt{\left(\frac{dx}{dt}\right)^2 + \left(\frac{dy}{dt}\right)^2}$, which implies that $ds/dt = 2$ for this problem, and where $\text{f}(x, y) = x^2 + y^2 + 3 = \cos^2(2t) + \sin^2(2t) + 3$, which equals 4. Thanks from zollen

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