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Find the particular solution of the differential equation (x^2/(y^2−2))(dx/dy)=1/(2y)?

satisfying the initial condition y(1)=3^(1/3).

Please show how the answer was solved so that I can figure out how to solve similar problems. Also, What type of differential equation is this? (1st order, 2nd order, ect)

1 Answer

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  • hfshaw
    Lv 7
    9 years ago
    Favorite Answer

    You have:

    (x^2/(y^2−2))(dx/dy)=1/(2y)

    (x^2)*dx/dy = (y^2 - 2)/(2y)

    This is a separable, (first-order) differential equation. Separating the variables:

    x^2 dx = (y/2 - 1/y) dy

    Integrate both sides:

    (x^3)/3) = (y^2)/4 - ln(y) + c

    where c is the constant of integration.

    x^3 = 3(y^2)/4 - 3*ln(y) + C

    where C = 3c is just another way of writing the constant.

    This is a general, implicit solution for y(x), and an explicit solution for x(y). There is no way to obtain an explit, closed-form solution for y(x) that can be expressed in terms of elementary functions.

    Now use the initial condition to find the solution to the initial value problem:

    If y(1) = 3^(1/3) then:

    1^3 = 3*(3^(2/3))/4 - 3*ln(3^(1/3)) + C

    1 = 3/4 - ln(3) + C

    C = ln(3) + 1/4

    So:

    x^3 = 3(y^2)/4 - 3*ln(y) + ln(3) + 1/4

    x^3 = (3/4)*y^2 + ln(3/y^3) + 1/4

    x(y) = [(3/4)*y^2 + ln(3/y^3) + 1/4] ^(1/3)

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