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The partial differential equation

\(u_{t} + u_{x} = u^{2}\)

can be classified as :

  • A.Linear
  • B.Non-linear
  • C.Semilinear
  • D.None of the above

Answer: C

The equation is semilinear.

A first-order PDE is semilinear when the derivatives appear linearly with coefficients depending only on the independent variables, while the remaining term may depend non-linearly on \(u\). In \(u_{t} + u_{x} = u^{2}\) the derivative part \(u_{t} + u_{x}\) is linear with constant coefficients, and the non-linearity is only in \(u^{2}\).

It is not linear, because of \(u^{2}\). It is non-linear in the broad sense, but the standard classification (linear, semilinear, quasilinear, fully non-linear) places it in the more specific class of semilinear equations.

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