Basic differential equations, homogeneous method

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SUMMARY

The discussion focuses on solving the differential equation xy' - y = sqrt(x^2 + y^2) using the homogeneous method. The user derives the equation arcsinh(y/x) = ln|x| + c but struggles to incorporate the relationship sinh(x) = 1/2 (e^x - e^-x) to reach the final solution. The expected answer is y = (1/2) * (cx^2 - 1/c). The conversation highlights the steps to manipulate the arcsinh function and apply the exponential identity to solve for y.

PREREQUISITES
  • Understanding of differential equations, specifically first-order equations.
  • Familiarity with the homogeneous method for solving differential equations.
  • Knowledge of hyperbolic functions, particularly sinh and arcsinh.
  • Ability to perform antiderivatives and manipulate logarithmic identities.
NEXT STEPS
  • Study the application of the homogeneous method in solving first-order differential equations.
  • Learn how to manipulate hyperbolic functions and their inverses in differential equations.
  • Explore the relationship between exponential functions and hyperbolic functions in depth.
  • Practice solving differential equations that involve square roots and hyperbolic identities.
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Students and educators in mathematics, particularly those studying differential equations, as well as anyone looking to deepen their understanding of hyperbolic functions and their applications in solving equations.

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xy'-y =sqrt((x^2)+y(^2))
so by method of homogeneous equations I get arcsinh(y/x) = ln|x|+c but don't know what to do from there. the book suggests using sinhx = 1/2 (e^x - e^-x) but I don't know how to incorporate this. The answer should be y= (1/2) * (cx^2 - 1/c). How do you get this?

This is my work so far:
y'-(y/x) = sqrt (1+(y/x)^2)
since y=ux...
du/(sqrt(1+u^2) = dx/x
taking the antiderivatives gives
arcsinh(u) = ln|x|+c
where
arcsinh|y/x| = ln|x|+c
 
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sinh^{-1}(\frac{y}{x}) = lnx + ln A (I put c=lnA to make things simpler)
\Rightarrow sinh^{-1}(\frac{y}{x}) = lnAx


\Rightarrow \frac{y}{x} = sinh(lnAx)


and they said that


sinhX=\frac{e^X - e^{-X}}{2}


So sinh(lnAx) should make the equation

\frac{y}{x}=\frac{e^{lnAx}-e^{-lnAx)}}{2}

Do you see how to continue from here?
 

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