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Example 10.45: Solving an Exponential Decay Application

To determine the future amount of a substance undergoing exponential decay, first find the decay constant kk using its half-life, and then use kk to calculate the final amount. For example, radium-226 has a half-life of 1,5901{,}590 years. If we start with a 100100-mg sample, at the half-life (t=1,590t = 1{,}590), there will be 5050 mg remaining (A=50A = 50). Using the exponential decay formula A=A0ektA = A_0 e^{kt}, substitute the known values: 50=100ek159050 = 100 e^{k \cdot 1590}. Dividing by 100100 gives 0.5=e1590k0.5 = e^{1590k}. Taking the natural logarithm of both sides yields ln0.5=1590k\ln 0.5 = 1590k, so the decay constant is k=ln0.51590k = \frac{\ln 0.5}{1590}. To find the amount of the sample left in 500500 years, substitute this rate and time back into the formula: A=100eln0.51590500A = 100 e^{\frac{\ln 0.5}{1590} \cdot 500}. Evaluating this expression yields A80.4A \approx 80.4 mg. Therefore, approximately 80.480.4 mg of radium-226 will remain after 500500 years.

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