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ExamsGATEEngineering Mathematics

\(f(z)=u(x,y)+iv(x,y)\) is an analytic function of the complex variable \(z=x+iy\), where \(i=\sqrt{-1}\). If \(u(x,y)=2xy\), then \(v(x,y)\) may be expressed as

  1. \(-x^2+y^2+\text{constant}\)
  2. \(x^2-y^2+\text{constant}\)
  3. \(x^2+y^2+\text{constant}\)
  4. \(-(x^2+y^2)+\text{constant}\)

Correct answer: \(-x^2+y^2+\text{constant}\)

Solution

For an analytic function, the Cauchy–Riemann equations give \(u_x=v_y\) and \(u_y=-v_x\). Since \(u=2xy\), we have \(u_x=2y\) and \(u_y=2x\), so \(v_y=2y\) and \(v_x=-2x\), which integrates to \(v=-x^2+y^2+C\).

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