Let us rewrite the differential equation as "\\frac{d e(t)}{d t} = - k e(t)", or "\\frac{d e(t)}{e(t)} = - k dt".
Integrating from both sides, obtain "\\ln \\frac{e(t)}{C} = - k t", from where "e(t) = C e^{-k t}".
Using initial condition, "e(0) = 4 = C", hence "e(t) = 4 e^{- k t}".
For "k=1", "e(2) = 4 e^{-2} \\approx 0.54".
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