Integral Question: Swapping X and Y

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SUMMARY

The discussion centers on the mathematical logic behind the equation involving the double integral of the function f(x, y) when x and y are swapped. Specifically, it establishes that the integral over the region defined by 0 to x and 0 to 1 is equal to half the integral over the unit square when f(x, y) is symmetric (i.e., f(x, y) = f(y, x)). The conclusion is that the integral over the triangular region is half of the integral over the entire unit square, confirming the relationship between the two integrals.

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Homework Statement


Can someone explain to me the logic of this statement:

"Since the value of f (x, y) is unchanged when we swap x with y,

[tex]\int_0^1 \int_0^x f (x+y)dydx = 1/2 \int_0^1 \int_0^1 f (x+y)dydx.[/tex]"

Homework Equations





The Attempt at a Solution



[tex]\int_0^1 \int_0^x f (x+y)dydx = \int_0^1 \int_0^y f (y+x)dxdy[/tex]

But I do not think that is the same.
 
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Draw a picture of the region of integration. It's one half of the unit square. What is the integral over the other half equal to if f(x,y)=f(y,x)?
 
That makes sense. :biggrin:
 

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