Difference between revisions of "Prototype Calculus Question"

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!Step 1:  
 
!Step 1:  
 
|-
 
|-
|Since we are rotating around the ''x''-axis, the washer method would utilize tall rectangles with ''dx'' as their width.  This seems like a reasonable choice, as these rectangles would be trapped between our two functions, allowing us to solve a single integral.
+
|'''Choosing the Approach:'''   Since we are rotating around the ''x''-axis, the washer method would utilize tall rectangles with ''dx'' as their width.  This seems like a reasonable choice, as these rectangles would be trapped between our two functions, allowing us to solve a single integral.
 
|}
 
|}
  
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!Step 2:  
 
!Step 2:  
 
|-
 
|-
|Since our rectangles will be trapped between the two functions, and will be rotated around the ''x''-axis (where <math> y=0 </math>), we find
+
|'''Finding the Radii:''' &nbsp;Since our rectangles will be trapped between the two functions, and will be rotated around the ''x''-axis (where <math> y=0 </math>), we find
 
|-
 
|-
 
|the inner radius is <math style="vertical-align:0%;"> r = 5-x </math>, represented by the blue line, while
 
|the inner radius is <math style="vertical-align:0%;"> r = 5-x </math>, represented by the blue line, while
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!Step 3: &nbsp;
 
!Step 3: &nbsp;
 
|-
 
|-
|We must set the two functions equal, and solve.  More to follow...
+
|'''Finding the Bounds of Integration:''' &nbsp; We must set the two functions equal, and solve.  If
 +
|-
 +
|<math>5-x=25-x^2,</math>
 +
|-
 +
|then
 +
|-
 +
|<math>x^2-x-20=(x+4)(x-20)=0,</math>
 +
|-
 +
|so we have roots -4 and 5. These are our bounds of integration.
 +
|}
 +
 
 +
{| class="mw-collapsible mw-collapsed" style = "text-align:left;"
 +
!Step 4: &nbsp;
 +
|-
 +
|'''Evaluating the Integral:''' &nbsp; Using the earlier steps, we have
 +
|-
 +
|<math>V=\pi \int_{-4}^{\, 5} (25-x^2)^2-(5-x)^2 \,dx</math>
 +
|-
 +
|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<math> =\pi \int_{-4}^{\, 5} 625-50x^2+x^4-(25-10x+x^2) \,dx</math>
 +
|-
 +
|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<math> =\pi \int_{-4}^{\, 5} 600-51x^2+x^4+10x \,dx</math>
 +
|-
 +
|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<math> =\pi \int_{-4}^{\, 5} 600-51x^2+x^4+10x \,dx</math>
 +
|-
 +
|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<math> =\pi \biggr(600x-51\cdot\frac{x^{3}}{3}+\frac{x^{5}}{5}+10\cdot\frac{x^{2}}{2}\biggr)\biggr|_{x=-4}^{5}</math>
 +
|-
 +
|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<math> =\frac{15,309}{5}\pi.</math>
 
|}
 
|}

Revision as of 15:43, 8 March 2015

9BSF1 3a.png

Find the volume of the solid obtained by rotating the area enclosed by and
around the x-axis.

Foundations  
• Choose either shell or washer method.
• Find the appropriate radii.
• Determine the bounds of integration by finding when both functions have the same y value.
• Using the determined values, set up and solve the integral.

Solution:

Step 1:  
Choosing the Approach:   Since we are rotating around the x-axis, the washer method would utilize tall rectangles with dx as their width. This seems like a reasonable choice, as these rectangles would be trapped between our two functions, allowing us to solve a single integral.
Step 2:  
Finding the Radii:  Since our rectangles will be trapped between the two functions, and will be rotated around the x-axis (where ), we find
the inner radius is , represented by the blue line, while
the outer radius is , represented by the red line.
Step 3:  
Finding the Bounds of Integration:   We must set the two functions equal, and solve. If
then
so we have roots -4 and 5. These are our bounds of integration.
Step 4:  
Evaluating the Integral:   Using the earlier steps, we have