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The solution of this integral equation is

  • A.\(\frac{x + 1}{1 - \lambda}\)
  • B.\(\frac{x^{2}}{1 - \lambda^{2}}\)
  • C.\(\frac{x}{1 + \lambda^{2}}\)
  • D.\(\frac{x}{1 - \lambda}\)

Answer: D

The solution is \(u(x) = \frac{x}{1 - \lambda}\).

The integral is a constant times \(x\): write \(u(x) = x(1 + \lambda C)\) with \(C = \int_{0}^{1}e^{t}u(t)\,dt\). Substituting back:

\[ \begin{aligned} C &= (1 + \lambda C)\int_{0}^{1}te^{t}\,dt = 1 + \lambda C \\ C &= \frac{1}{1 - \lambda} \\ u(x) &= x\left(1 + \frac{\lambda}{1 - \lambda}\right) = \frac{x}{1 - \lambda} \end{aligned} \]

The same follows from the resolvent of Q.50: \(u = x + \lambda\int_{0}^{1}\frac{xe^{t}}{1-\lambda}t\,dt\), which is \(x + \frac{\lambda x}{1-\lambda}\). \(\frac{x+1}{1-\lambda}\) adds a constant the equation cannot produce, since every term is a multiple of \(x\); \(x^{2}\) cannot appear for the same reason; and \(\frac{x}{1+\lambda^{2}}\) does not satisfy the equation.

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