Raining in Flatland

Calculus Level 3

In flatland, everything is two-dimensional. There is a container with the shape of a curve

f ( x ) = x 3 5 x 2 + 7 x . f(x)={ x }^{ 3 }-5{ x }^{ 2 }+7x.

If gravitational force is parallel to y y -axis, what is the maximum amount of water it can contain?

Give your answer to 3 decimal places.


The answer is 1.333.

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1 solution

Rishabh Jain
Mar 12, 2016

Drawing the graph: F ( x ) = x ( x 2 5 x + 7 ) F(x)=x(x^2-5x+7) Since discriminant of x 2 5 x + 7 x^2-5x+7 is negative therefore graph will intersect x axis only at origin. F ( x ) = 3 x 2 10 x + 7 = ( 3 x 7 ) ( x 1 ) F'(x)=3x^2-10x+7=(3x-7)(x-1) \implies graph is increasing in ( , 1 ) (-\infty,1) and ( 7 3 , ) (\dfrac 73,\infty) while its decreasing in ( 1 , 7 3 ) (1,\dfrac 73) hence attaining a local maxima at x = 1 x=1 and a local minima at x = 7 3 x=\dfrac 73 along with having horizontal tangents at x = 1 x=1 and x = 7 3 x=\dfrac 73 . Hence the graph would look like. Clearly required region is : 1 3 ( 3 ( x 3 5 x 2 + 7 x ) ) d x \int_1^3\left(3-(x^3-5x^2+7x)\right)\, dx = 4 3 1.333 =\Large \dfrac 43\approx \boxed{1.333}

Why is it equal to the amount of water it will contain?

Anik Mandal - 5 years, 3 months ago

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Because everything is in 2-D I guess..

Rishabh Jain - 5 years, 3 months ago

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