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For exponential decay (radioactive): dN/dt = -λN. Solution:
AN = N₀ e^(λt) (growth)
BN(t) = 0
CN = -λt
DN = N₀ e^(-λt) (decay)
Answer & Solution
Correct answer: D. N = N₀ e^(-λt) (decay)
dN/N = -λ dt → ln N = -λ t + C → N = N₀ e^(-λt). Half-life t₁/₂ = ln 2/λ. Models radioactive decay, drug elimination, RC circuit discharge.
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