imo class 12 differential equations

A substance undergoes radioactive decay according to dN/dt = −λN. If N₀ is the initial amount at t=0, the amount N at time t is:

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A substance undergoes radioactive decay according to dN/dt = −λN. If N₀ is the initial amount at t=0, the amount N at time t is:

  • A. N₀ e^(λt)
  • B. N₀ e^(−λt)
  • C. N₀ + e^(−λt)
  • D. N₀ − λt

Answer: B) N₀ e^(−λt)

Explanation: dN/N = −λ dt. Integrating gives log N = −λt + c. At t=0, N=N₀, so c = log N₀. log(N/N₀) = −λt, which yields N = N₀ e^(−λt).

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