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Ch. 9 - First-Order Differential Equations
Hass - Thomas' Calculus 15th Edition
Hass15th EditionThomas' CalculusISBN: 9780137616077Not the one you use?Change textbook
Chapter 9, Problem 9.AAE.9

Solve the homogeneous equations in Exercises 5–10. First put the equation in the form of a homogeneous equation.


y' = y/x + cos ((y-x)/x)

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1
Rewrite the given differential equation: \(y' = \frac{y}{x} + \cos\left(\frac{y - x}{x}\right)\).
Recognize that the equation involves expressions of \(y\) and \(x\) in the form \(\frac{y}{x}\) and \(\frac{y - x}{x}\), which suggests a substitution using \(v = \frac{y}{x}\) to transform it into a homogeneous equation.
Express \(y\) in terms of \(v\) and \(x\): \(y = vx\). Then, differentiate both sides with respect to \(x\) using the product rule: \(y' = v + x \frac{dv}{dx}\).
Substitute \(y = vx\) and \(y' = v + x \frac{dv}{dx}\) back into the original equation to rewrite it entirely in terms of \(v\) and \(x\): \(v + x \frac{dv}{dx} = v + \cos(v - 1)\).
Simplify the equation by canceling terms and isolate \(\frac{dv}{dx}\) to get a separable differential equation in \(v\) and \(x\), which can then be solved by integration.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Homogeneous Differential Equations

A differential equation is homogeneous if it can be expressed so that the right-hand side is a function of the ratio y/x alone. This allows substitution like v = y/x to simplify the equation into a separable form, making it easier to solve.
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Substitution Method (v = y/x)

By substituting v = y/x, the original variables y and x are transformed into a single variable v, reducing the differential equation to one involving v and x. This substitution leverages the homogeneity property to simplify the equation.
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Separable Differential Equations

Once the equation is rewritten in terms of v and x, it often becomes separable, meaning it can be expressed as a product of a function of v and a function of x. This allows integration on both sides to find the general solution.
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