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Math Help - Multiple integrals help

  1. #1
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    Multiple integrals help

    1) The integral (between 4 and 1) of the integral (between 2 and 1) (x/y+y/x)dydx
    2)The integral (between pi/3 and 0) of the integral (between pi/6 and 0)
    xsin(x+y)dydx

    Thanks!
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  2. #2
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    Quote Originally Posted by daskywalker View Post
    1) The integral (between 4 and 1) of the integral (between 2 and 1) (x/y+y/x)dydx
    2)The integral (between pi/3 and 0) of the integral (between pi/6 and 0)
    xsin(x+y)dydx

    Thanks!

    1.- \int\limits_1^4\int\limits_1^2 \frac{x}{y}+\frac{y}{x}\;dy\,dx=\int\limits_1^4dx\  left(\int\limits_1^2\left(\frac{x}{y}+\frac{y}{x}\  right)dy\right)=\int\limits_1^4dx\left(x\ln y+\frac{1}{2x}y^2\right)_1^2

    Well, I did the first one now you do the second one AFTER plugging in the limits (for y!) in the primitive function we got above. You get two nice, not hard integrals in x.

    Try to walk now your way with problem 2 and if you get stuck somewhere in the way write back AFTER you do some work on it.

    Tonio
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  3. #3
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    Quote Originally Posted by tonio View Post
    1.- \int\limits_1^4\int\limits_1^2 \frac{x}{y}+\frac{y}{x}\;dy\,dx=\int\limits_1^4dx\  left(\int\limits_1^2\left(\frac{x}{y}+\frac{y}{x}\  right)dy\right)=\int\limits_1^4dx\left(x\ln y+\frac{1}{2x}y^2\right)_1^2

    Well, I did the first one now you do the second one AFTER plugging in the limits (for y!) in the primitive function we got above. You get two nice, not hard integrals in x.

    Try to walk now your way with problem 2 and if you get stuck somewhere in the way write back AFTER you do some work on it.

    Tonio
    ok got 1 now...almost got 2, I am having trouble with a +- sign somewhere...

    so for 2 I got [-xcos(x+pi/3)]-[-xcos(x)] (after I evaluated the first integral between pi/3 and 0)

    integrate it becomes [-xsin(x+pi/3)+cos(x+pi/3)+cos(x)+xsin(x)]
    evaluate it between pi/6 and 0 I get (-pi/12)+0.5[sqrt(3)-3], but the correct answer is (-pi/12)+0.5[sqrt(3)-1]
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