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The condition for \( M\,dx + N\,dy = 0 \) to be exact is

  • A.\( \partial M/\partial y = -\partial N/\partial x \)
  • B.\( \partial M/\partial y = \partial N/\partial x \)
  • C.\( \partial M/\partial x = \partial N/\partial y \)
  • D.\( M = N \) throughout the region

Answer: B

Exactness means the expression is \( d\phi \) for some \( \phi \), with \( M = \phi_{x} \) and \( N = \phi_{y} \). Equality of the mixed second derivatives then requires

\[ \frac{\partial M}{\partial y} \;=\; \frac{\partial N}{\partial x}. \]

The cross pairing is what matters: comparing each with the derivative in its own variable tests nothing about a potential

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