# Math Help - Double intergral

1. ## Double intergral

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I usually know how to do double integrals, but i have no idea how to deal with this one. plz help me solve/understand this.

2. Originally Posted by DMDil
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I usually know how to do double integrals, but i have no idea how to deal with this one. plz help me solve/understand this.
The integrand is seperable in the variables: $x e^x y e^y$. So it becomes the product of two seperate integrals, each of which can be done using integration by parts.

3. Originally Posted by DMDil
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I usually know how to do double integrals, but i have no idea how to deal with this one. plz help me solve/understand this.
integrate with respect to y first and then with respect to x, plugging in the limits as you go. note, when integrating with respect to y, treat x as a constant, and vice versa.

you need integration by parts here. to start off, let $u = xy$ and $dv = e^{x + y}~dy$ (remember, x is a constant!)

Originally Posted by mr fantastic
The integrand is seperable in the variables: $x e^x y e^y$. So it becomes the product of two seperate integrals, each of which can be done using integration by parts.
hehe, even nicer

4. Originally Posted by DMDil
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I usually know how to do double integrals, but i have no idea how to deal with this one. plz help me solve/understand this.
Since you're on a rectangle and the expontential spilts up then

$\int_4^7 x e^x dx \int_5^7 y e^y dy$ then parts on each part.