Volume Integral of xy over Triangle Area

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In summary: Yesx and 2x-2You must learn to check your own work. For ##y = 2x - 2##, when ##x = 0##, ##y = -2##. That's clearly wrong. You should check both points you have lie on that line.
  • #1
physicss
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Homework Statement
Hello, the homework statement is: calculate the 2d volume integral over the area defined by the triangle with the vertices: (0,0), (0,1)
and (2,2).
of the function xy
Relevant Equations
(0,0), (0,1)
and (2,2).
My solution is 2. would that be correct? I did use double Integrals
 
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  • #2
How about showing us your solution? Then we can tell you if it is correct and if not, where you went wrong.
 
  • #3
kuruman said:
How about showing us your solution? Then we can tell you if it is correct and if not, where you went wrong.
∬(R) xy dA = ∫(0 to 2) ∫(0 to x) xy dy dx

= ∫(0 to 2) [x^2 y/2]_0^x dx

= ∫(0 to 2) (1/2) x^3 dx

(1/8) x^4 |_0^2

= (1/8) (2^4 - 0)= 2
 
  • #4
See picture below for the triangle of interest. You need the equation for the line AB.

IntegrationTriangle.png


(Edited to fix the figure as pointed out in post #5.)
 
Last edited:
  • #5
kuruman said:
The lower limit for ##y## is zero only when ##x\leq 1##. See picture below for the triangle of interest. You need the equation for the line AB.

View attachment 328334
Point A should be ##(0, 1)##, according to the OP.
 
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  • #6
Oops. I swapped coordinates in my head.
 
  • #7
physicss said:
∬(R) xy dA = ∫(0 to 2) ∫(0 to x) xy dy dx
##x \in [0, 2], \ y \in [0, x]## represents the triangle with vertices ##(0,0), (2, 0), (2, 2)##.
 
  • #8
PeroK said:
##x \in [0, 2], \ y \in [0, x]## represents the triangle with vertices ##(0,0), (2, 0), (2, 2)##.
could I also calculate it by forming a rectangle?

∫∫R xy dA = ∫∫S u(u+v) dudv
∫∫S u(u+v) dudv = ∫0^2 ∫0^1 u(u+v) dvdu
= ∫0^2 [(u^2v/2) + (uv^2/2)]_0^1 du
= ∫0^2 (u^2/2 + u/2) du
= [(u^3/6) + (u^2/4)]_0^2
= 2.
 
  • #9
physicss said:
could I also calculate it by forming a rectangle?

∫∫R xy dA = ∫∫S u(u+v) dudv
∫∫S u(u+v) dudv = ∫0^2 ∫0^1 u(u+v) dvdu
= ∫0^2 [(u^2v/2) + (uv^2/2)]_0^1 du
= ∫0^2 (u^2/2 + u/2) du
= [(u^3/6) + (u^2/4)]_0^2
= 2.
I've no idea what you are doing there. You need to sort out the correct bounds for your integral.
 
  • #10
PS the answer is not 2.
 
  • #11
PeroK said:
PS the answer is not 2.
Hello, I recalculated it ( in a shorter way). I still get 2:

∫(0 to 2) (∫(0 to y) xy dx) dy = ∫(0 to 2) (y^3)/2 dy =2

what am I doing wrong?

thanks in advance
 
  • #12
physicss said:
Hello, I recalculated it ( in a shorter way). I still get 2:

∫(0 to 2) (∫(0 to y) xy dx) dy = ∫(0 to 2) (y^3)/2 dy =2

what am I doing wrong?

thanks in advance
Your bounds are still wrong. You are including the area above the line AB in post #4.
For a given x, what is the range of y within the triangle?
 
  • #13
physicss said:
Hello, I recalculated it ( in a shorter way). I still get 2:

∫(0 to 2) (∫(0 to y) xy dx) dy = ∫(0 to 2) (y^3)/2 dy =2

what am I doing wrong?

thanks in advance
Now you've integrated over the triangle with vertices at ##(0,0), (0, 2), (2, 2)## .

Use the figure given by @kuruman

kuruman said:
See picture below for the triangle of interest. You need the equation for the line AB.

integrationtriangle-png.png


(Edited to fix the figure as pointed out in post #5.)
What is the equation of the line passing through O and B ?

What is the equation of the line passing through A and B ?
 
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  • #14
SammyS said:
Now you've integrated over the triangle with vertices at ##(0,0), (0, 2), (2, 2)## .

Use the figure given by @kurumanWhat is the equation of the line passing through O and B ?

What is the equation of the line passing through A and B ?
x and 2x-2
 
  • #15
physicss said:
2x-2
y=2x-2? No. Try plotting that on the figure... :wink:
 
  • #16
berkeman said:
y=2x-2? No. Try plotting that on the figure... :wink:
Thanks, while writing down I swapped x and y. 0.5x+1 is AB
 
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  • #17
Great. So how does that change your integrals? :smile:
 
  • #18
berkeman said:
Great. So how does that change your integrals? :smile:
the inner integral has to go from x to 0.5x+1 xy dy and the outer from 0 to 2 dx I guess?
 
  • #19
physicss said:
the inner integral has to go from x to 0.5x+1 xy dy and the outer from 0 to 2 dx I guess?
Yes
 
  • #20
physicss said:
x and 2x-2
You must learn to check your own work. For ##y = 2x - 2##, when ##x = 0##, ##y = -2##. That's clearly wrong. You should check both points you have lie on that line.
 

FAQ: Volume Integral of xy over Triangle Area

What is a volume integral of xy over a triangle area?

A volume integral of xy over a triangle area refers to the double integral of the function f(x, y) = xy over a triangular region in the xy-plane. This involves calculating the integral of xy with respect to both x and y over the specified triangular domain.

How do you set up the integral for the volume integral of xy over a triangular area?

To set up the integral, you need to define the limits of integration based on the vertices of the triangle. For a triangle with vertices at (x1, y1), (x2, y2), and (x3, y3), you express the integral as a double integral: ∫∫_T xy dA. The limits for x and y will depend on the linear equations of the triangle's sides.

What are the steps to solve the volume integral of xy over a triangular area?

The steps to solve the volume integral are: 1) Determine the equations of the lines forming the triangle's sides, 2) Set up the double integral with appropriate limits for x and y, 3) Integrate with respect to y first, then x (or vice versa), 4) Evaluate the resulting integral to find the volume.

Can you provide an example of a volume integral of xy over a specific triangle?

Sure, consider a triangle with vertices at (0,0), (1,0), and (0,1). The integral setup would be ∫ from 0 to 1 ∫ from 0 to 1-x (xy dy dx). First, integrate xy with respect to y from 0 to 1-x, then integrate the resulting expression with respect to x from 0 to 1.

What are some common applications of volume integrals of xy over triangular areas?

Volume integrals of xy over triangular areas are used in various fields such as physics, engineering, and computer graphics. They are particularly useful in calculating moments of inertia, areas under surfaces, and in finite element analysis for solving partial differential equations.

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