If the diameter of a circle is 80 ft., find the area A 5,024 ft2 B 20,096 ft2 C 251.2 ft2 D 125.6 ft2
1 answer:
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(a) See the attached sketch. Each shell will have a radius <em>y</em> chosen from the interval [2, 4], a height of <em>x</em> = 2/<em>y</em>, and thickness ∆<em>y</em>. For infinitely many shells, we have ∆<em>y</em> converging to 0, and each super-thin shell contributes an infinitesimal volume of
2<em>π</em> (radius)² (height) = 4<em>πy</em>
Then the volume of the solid is obtained by integrating over [2, 4]:
(b) See the other attached sketch. (The text is a bit cluttered, but hopefully you'll understand what is drawn.) Each shell has a radius 9 - <em>x</em> (this is the distance between a given <em>x</em> value in the orange shaded region to the axis of revolution) and a height of 8 - <em>x</em> ³ (and this is the distance between the line <em>y</em> = 8 and the curve <em>y</em> = <em>x</em> ³). Then each shell has a volume of
2<em>π</em> (9 - <em>x</em>)² (8 - <em>x</em> ³) = 2<em>π</em> (648 - 144<em>x</em> + 8<em>x</em> ² - 81<em>x</em> ³ + 18<em>x</em> ⁴ - <em>x</em> ⁵)
so that the overall volume of the solid would be
I leave the details of integrating to you.
Answer:
Part A)
The <em>x-</em>intercepts are (-1/4, 0) and (4, 0).
Part B)
The vertex is a maximum because the leading coefficient is negative.
The vertex is (1.875, 72.25).
Part C)
We can plot the zeros and the vertex, and connect them with a curve.
Step-by-step explanation:
The function given is:
Part A)
To find the <em>x-</em>intercepts of the function, set the function equal to 0 and solve for <em>x: </em>
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We can divide both sides by negative four:
Factor:
Zero Product Property:
Solve for each case:
Hence, our zeros are:
Part B)
Note that the leading coefficient of our function is negative.
So, our function will be concave down.
Hence, our vertex will be the maximum.
The vertex is given by:
In this case, <em>a </em>= -16, <em>b</em> = 60, and <em>c</em> = 16.
Find the <em>x-</em>coordinate of the vertex:
Substitute this back into the function to find the <em>y-</em>coordinate:
Hence, our vertex is:
Part C)
Since we already determined the zeros and the vertex, we can plot the two zeros and the vertex and draw a curve between the three points.
The graph is shown below. Again, to do this by hand, simply plot the three points and connect them with a parabola. If necessary, we can also find the <em>y-</em>intercept.
