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The Volterra integral equation

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

is equivalent to the initial value problem :

  • A.\(x''(t) + x(t) = 0\), \(x(0) = 0\), \(x'(0) = 1\)
  • B.\(x''(t) - x(t) = 0\), \(x(0) = 1\), \(x'(0) = 0\)
  • C.\(x''(t) + x'(t) = 0\), \(x(0) = 1\), \(x'(0) = 0\)
  • D.\(x''(t) - x'(t) = 0\), \(x(0) = 0\), \(x'(0) = 1\)

Answer: A

The equivalent problem is x'' + x = 0, x(0) = 0, x'(0) = 1.

Differentiate \(x(t) = t + \int_{0}^{t}(s - t)x(s)\, ds\) using Leibniz's rule:

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

At \(t = 0\) the integrals vanish, so \(x(0) = 0\) and \(x'(0) = 1\). The solution is \(x = \sin t\).

The options with \(x'' - x\) have the wrong sign, those with \(x'\) in place of x differentiate wrongly, and \(x(0) = 1,\ x'(0) = 0\) swaps the initial values.

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