Let us rewrite the differential equation as de(t)dt=−ke(t)\frac{d e(t)}{d t} = - k e(t)dtde(t)=−ke(t), or de(t)e(t)=−kdt\frac{d e(t)}{e(t)} = - k dte(t)de(t)=−kdt.
Integrating from both sides, obtain lne(t)C=−kt\ln \frac{e(t)}{C} = - k tlnCe(t)=−kt, from where e(t)=Ce−kte(t) = C e^{-k t}e(t)=Ce−kt.
Using initial condition, e(0)=4=Ce(0) = 4 = Ce(0)=4=C, hence e(t)=4e−kte(t) = 4 e^{- k t}e(t)=4e−kt.
For k=1k=1k=1, e(2)=4e−2≈0.54e(2) = 4 e^{-2} \approx 0.54e(2)=4e−2≈0.54.
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