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Homogeneous & Linear Differential Equations · Question 54 of 120

Question 54

HardOriginal question

What is the general solution of dydx=yx+tan⁡yx\frac{dy}{dx} = \frac{y}{x} + \tan\frac{y}{x}?

Options for question 54

Show the answer and explanation
CorrectNot quiteAnswer D

Correct answer: D

Explanation

  1. y=vxy = vx: v+xdvdx=v+tan⁡vv + x\frac{dv}{dx} = v + \tan v, so cot⁡v dv=dxx\cot v\,dv = \frac{dx}{x}.

  2. log⁡∣sin⁡v∣=log⁡∣x∣+c\log\lvert \sin v \rvert = \log\lvert x \rvert + c, so sin⁡yx=Cx\sin\frac{y}{x} = Cx.

Worth noting

cos⁡yx=Cx\cos\frac{y}{x} = Cx takes ∫cot⁡v dv\int \cot v\,dv as log⁡∣cos⁡v∣\log\lvert \cos v \rvert; that is −∫tan⁡v dv-\int \tan v\,dv.

Concept
Solving a homogeneous equation
Typical time
120 s
Your time
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The rule this testsSolving homogeneous equations
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Ten questions from Homogeneous & Linear Differential Equations, at your own pace.

Practise this chapter →

Ten questions from Homogeneous & Linear Differential Equations, at your own pace.