Curvy Calculus 2

Calculus Level 5

x = 0 y = 2 + 2 cos t z = 2 + 2 sin t } 0 t 2 π \left.\begin{matrix} & x=0 & \\ & y=2+2\cos t & \\ & z=2+2\sin t & \end{matrix}\right\}0\leq t\leq 2\pi

Let C C be the curve defined by parametric equations above.

Then evaluate C x 2 e 5 z d x + x cos y d y + 3 y d z \int_{C}x^{2}e^{5z}dx + x \cos y dy+3y dz

Answer is of the form A π A\pi submit the answer as A A .


The answer is 12.

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2 solutions

Otto Bretscher
Feb 19, 2016

The parametrisation gives 0 2 π ( 0 + 0 + 3 ( 2 + 2 cos t ) 2 cos t ) d t = 12 0 2 π cos 2 t d t \int_{0}^{2\pi}(0+0+3(2+2\cos t)2\cos t)dt=12\int_{0}^{2\pi}\cos^2 t dt = 12 π =\boxed{12}\pi

Alexander Koran
Jul 14, 2019

C x 2 e 5 z d x + x cos y d y + 3 y d z \int_{C} x^2e^{5z}dx+x \cos y dy + 3y dz

= 0 2 π ( x 2 e 5 z d x d t + x cos y d y d t + 3 y d z d t ) d t = \int_{0}^{2 \pi} (x^2e^{5z} \frac{dx}{dt}+x \cos y \frac{dy}{dt} + 3y \frac{dz}{dt}) dt

= 0 2 π ( ( 0 ) 2 e 5 z d x d t + 0 cos y d y d t + 3 y d z d t ) d t = \int_{0}^{2 \pi} ((0)^2 \cdot e^{5z} \frac{dx}{dt}+0 \cdot \cos y \frac{dy}{dt} + 3y \frac{dz}{dt}) dt

= 0 2 π 3 ( 2 + 2 cos t ) 2 cos t d t = \int_{0}^{2 \pi} 3 (2+2 \cos t) \cdot 2 \cos t dt

= 12 0 2 π cos 2 t + cos t d t = 12 \int_{0}^{2 \pi} \cos ^2 t + \cos t dt

= 12 0 2 π 1 + cos 2 t 2 + cos t d t = 12 \int_{0}^{2 \pi} \frac{1+ \cos 2t}{2} + \cos t dt

= 12 1 2 t + 1 4 sin 2 t + sin t 0 2 π = 12 | \frac{1}{2} t +\frac{1}{4} \sin 2t + \sin t |_{0}^{2 \pi}

= 12 π A = 12 = 12 \pi \implies A = \boxed{12}

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