Cardiod-oid

Calculus Level 5

Find the surface area of the body generated by the revolution around the x x axis of the curve defined by the parametric equations x = ( 1 + cos ( t ) ) cos ( t ) , y = ( 1 + cos ( t ) ) sin ( t ) x=(1+\cos(t))\cos(t),\quad y=(1+\cos(t))\sin(t) for 0 t 2 π 0\leq t\leq 2\pi .

If the surface area can be expressed as A B π \dfrac{A}{B} \pi , where A A and B B are coprime positive integers, submit your answer as A + B A+B .

Bonus : Find the tangent plane to the body defined above through the point ( 1 + 2 2 , 1 + 2 2 , 0 ) (\frac{1+\sqrt{2}}{2},\frac{1+\sqrt{2}}{2},0) .


The answer is 37.

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1 solution

Mark Hennings
Jan 19, 2018

We are being asked for the surface of revolution of the cardioid r = 1 + cos θ r = 1 + \cos\theta , which is S = 2 π 0 π r sin θ r 2 + ( d r d θ ) 2 d θ = 2 π 0 π ( 1 + cos θ ) sin θ 2 ( 1 + cos θ ) d θ = 2 π 2 0 π ( 1 + cos θ ) 3 2 sin θ d θ = 2 π 2 [ 2 5 ( 1 + cos θ ) 5 2 ] 0 π = 32 5 π \begin{aligned} S & = \; 2\pi\int_0^\pi r\sin\theta \sqrt{r^2 + \left(\frac{dr}{d\theta}\right)^2}\,d\theta \; = \; 2\pi\int_0^\pi (1 + \cos\theta)\sin\theta \sqrt{2(1 + \cos\theta)}\,d\theta \\ & = \; 2\pi\sqrt{2}\int_0^\pi (1 + \cos\theta)^{\frac32}\sin\theta\,d\theta \; = \; 2\pi\sqrt{2}\Big[-\tfrac25(1 + \cos\theta)^{\frac{5}{2}}\Big]_0^\pi \; = \; \tfrac{32}{5}\pi \end{aligned} making the answer 32 + 5 = 37 32+5=\boxed{37} .

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