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The following partial differential equation :

\[ \begin{aligned} &(x^{2} + z^{2})\frac{\partial z}{\partial x} - xy\frac{\partial z}{\partial y} \\ &= z^{3}x + y^{2} \end{aligned} \]

is :

  • A.linear
  • B.non-linear
  • C.semi-linear
  • D.Quasi-linear

Answer: D

The equation is quasi-linear.

A first-order equation \(a\,z_{x} + b\,z_{y} = c\) is quasi-linear when it is linear in the derivatives \(z_{x}, z_{y}\) but the coefficients \(a, b\) may depend on \(z\) itself. Here the coefficient of \(z_{x}\) is \(x^{2} + z^{2}\), which contains \(z\), and the derivatives appear only to the first power.

It is not linear or semi-linear, because those need \(a\) and \(b\) to depend on \(x, y\) only. It is not fully non-linear, since there are no products or powers of the derivatives. The \(z^{3}\) on the right is allowed in a quasi-linear equation.

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