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The solution of the integral equation

\[ x(t) = t + \int_{0}^{t} (s - t)\, x(s)\, ds \]

with the help of resolvent kernel is :

  • A.\(x(t) = \cos t\)
  • B.\(x(t) = \sin t\)
  • C.\(x(t) = e^{t}\)
  • D.\(x(t) = 1\)

Answer: B

The solution is \(x(t) = \sin t\).

The kernel is \(s - t = -(t - s)\). The resolvent route gives the same result as differentiating, which is quicker:

\[ \begin{aligned} x(t) &= t - \int_{0}^{t} (t - s)\, x(s)\, ds \\ x'(t) &= 1 - \int_{0}^{t} x(s)\, ds \\ x''(t) &= -x(t) \end{aligned} \]

with \(x(0) = 0\) and \(x'(0) = 1\). The solution is \(\sin t\). (The resolvent kernel of \(-(t - s)\) is \(-\sin(t - s)\), and \(x = t - \int_{0}^{t}\sin(t - s)\, s\, ds = \sin t\).)

\(\cos t\), \(e^{t}\) and 1 all fail \(x(0) = 0\), which the equation forces at \(t = 0\) because the integral vanishes there.

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