Find the Volume.

Calculus Level 4

Let b b and c c be real constants. Minimize the volume of the region bounded between y = x 4 + b x 2 + c y = x^4 + bx^2 + c , x = 0 x= 0 and x = 1 x=1 , when it is revolved about the x x -axis.

If this volume can be expressed as m n π \dfrac mn \pi , where m m and n n are coprime positive integers, submit your answer as m + n m+n .


The answer is 11089.

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1 solution

Rocco Dalto
Apr 5, 2018

The volume V = π 0 1 ( x 4 + b x 2 + c ) 2 d x = π 0 1 ( x 8 + 2 b x 6 + ( b 2 + 2 c ) x 4 + 2 b c x 2 + c 2 ) d x V = \pi\int_{0}^{1} (x^4 + bx^2 + c)^2 dx = \pi\int_{0}^{1} (x^8 + 2bx^6 + (b^2 + 2c)x^4 + 2bc x^2 + c^2)dx

= π ( x 9 9 + 2 b 7 x 7 + b 2 + 2 c 5 x 5 + 2 b c 3 x 3 + c 2 x ) 0 1 = = \pi(\dfrac{x^9}{9} + \dfrac{2b}{7}x^7 + \dfrac{b^2 + 2c}{5}x^5 + \dfrac{2bc}{3}x^3 + c^2x)_{0}^{1} = π ( 1 9 + 2 b 7 + b 2 + 2 c 5 + 2 b c 3 + c 2 ) \pi(\dfrac{1}{9} + \dfrac{2b}{7} + \dfrac{b^2 + 2c}{5} + \dfrac{2bc}{3} + c^2)

V b = π ( 2 7 + 2 b 5 + 2 c 3 ) = 0 \implies \dfrac{\partial{V}}{\partial{b}} = \pi(\dfrac{2}{7} + \dfrac{2b}{5} + \dfrac{2c}{3}) = 0 and V c = π ( 2 5 + 2 b 3 + 2 c ) = 0 \dfrac{\partial{V}}{\partial{c}} = \pi(\dfrac{2}{5} + \dfrac{2b}{3} + 2c) = 0

1 5 b + 1 3 c = 1 7 \implies \dfrac{1}{5}b + \dfrac{1}{3}c = \dfrac{-1}{7} and 1 3 b + c = 1 5 \dfrac{1}{3}b + c = \dfrac{-1}{5}

Solving the system we obtain b = 6 7 b = \dfrac{-6}{7} and c = 3 35 c = \dfrac{3}{35} .

2 V b 2 = 2 π 5 > 0 \dfrac{\partial^2{V}}{\partial{b^2}} = \dfrac{2\pi}{5} > 0 and 2 V c 2 = 2 π > 0 \dfrac{\partial^2{V}}{\partial{c^2}} = 2\pi > 0 and b ( V c ) = c ( V b ) = 2 π 3 \dfrac{\partial}{\partial{b}}(\dfrac{\partial{V}}{\partial{c}}) = \dfrac{\partial}{\partial{c}}(\dfrac{\partial{V}}{\partial{b}}) = \dfrac{2\pi}{3}

The hessian matrix M = 2 π 5 2 π 3 2 π 3 2 π M= \begin{vmatrix}{\dfrac{2\pi}{5}} && {\dfrac{2\pi}{3}} \\ {\dfrac{2\pi}{3}} && {2\pi}\end{vmatrix} and det ( M ) > 0 \det(M) > 0 \implies we have a minimum at ( 6 7 , 3 35 ) (\dfrac{-6}{7},\dfrac{3}{35}) \implies

V m i n = π ( 1225 2700 + 1998 540 + 81 11025 ) = 64 11025 π = m n π V_{min} = \pi(\dfrac{1225 - 2700 + 1998 -540 + 81}{11025}) = \dfrac{64}{11025}\pi = \dfrac{m}{n}\pi

m + n = 11089 \implies m + n = \boxed{11089} .

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