Paraboloid Volume

Calculus Level 5

z = x 2 4 + y 2 z = \dfrac{x^2}{4} + y^2

The paraboloid given by the equation above is cut by the plane

z = 1 4 x + 1. z = \dfrac{1}{4} x + 1.

Find the volume of the region bounded by the paraboloid and the plane. If the volume can be written as V = ( a b ) 2 π V = \left( \dfrac ab\right)^2 \pi , where a , b a, b are coprime positive integers , enter a + b a + b as your answer.


The answer is 33.

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1 solution

Mark Hennings
Aug 2, 2016

The volume is V = x 2 + 4 y 2 x + 4 [ 1 + 1 4 x ( 1 4 x 2 + y 2 ) ] d x d y = ( x 1 2 ) 2 + 4 y 2 17 4 [ 17 16 1 4 ( x 1 2 ) 2 y 2 ] d x d y V \;=\; \iint_{x^2 + 4y^2 \le x + 4} \Big[1 + \tfrac14x - \big(\tfrac14x^2 + y^2\big)\Big]\,dxdy \; = \; \iint_{(x-\frac12)^2 + 4y^2 \le \frac{17}{4}} \Big[\tfrac{17}{16} - \tfrac14(x - \tfrac12)^2 - y^2\Big]\,dxdy Parametrizing the region of integration by x = 1 2 + r cos θ y = 1 2 r sin θ 0 r 17 2 , 0 θ 2 π x = \tfrac12 + r\cos\theta \qquad y \; = \; \tfrac12r\sin\theta \qquad \qquad 0 \le r \le \frac{\sqrt{17}}{2}\,,\,0 \le \theta \le 2\pi we see that V = 1 2 0 2 π d θ 0 17 2 ( 17 16 1 4 r 2 ) r d r = 289 256 π V \; = \; \tfrac12 \int_0^{2\pi} d\theta \int_0^{\frac{\sqrt{17}}{2}} \left(\tfrac{17}{16} - \tfrac14r^2\right)\,r\,dr \; = \; \frac{289}{256}\pi making the answer 17 + 16 = 33 17+16 = \boxed{33} .

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