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Solution of the differential equation

\(xdy - ydx = 0,\)

represents a :

  • A.straight line
  • B.hyperbola
  • C.parabola
  • D.circle

Answer: A

The solution represents a straight line.

Separate the variables:

\[ \begin{aligned} \frac{dy}{y} &= \frac{dx}{x} \\ \log|y| &= \log|x| + C \\ y &= cx \end{aligned} \]

The general solution is the family of straight lines through the origin. (Equivalently, \(x\,dy - y\,dx = 0\) says \(d(y/x) = 0\).) A hyperbola would come from \(x\,dy + y\,dx = 0\), giving \(xy = c\), and a circle from \(x\,dx + y\,dy = 0\).

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