Let \(f : [0, \infty ) \to \mathbb {R}\) defined by \(f(x) = e^{-x}\). Explain why \(f\) is Lebesgue integrable and compute its
integral. You may use the fact that if a function \(f : [0, \infty ) \to \mathbb {R}\) is 0 outside an interval \([a, b]\)
and \(f|_{[a,b]} : [a, b] \to \mathbb {R}\) is Riemann integrable then \(f\) is Lebesgue integrable and the Lebesgue
integral of \(f\) equals the Riemann integral of \(f|_{[a,b]}\).