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  • Perplex
    IB Math AAHL
    /
    Exponents & Logarithms
    /

    Problem Bank

    [Maximum mark: 15]

    Samples of ​2​ radioactive isotopes, each with initial mass ​1kg, are prepared in the Perplex Laboratory. Over time, the mass of the samples will decrease due to radioactive decay. The remaining mass, in ​kg, of each sample after ​t​ years can be modeled by the functions

    ​
    {A(t)=e−atB(t)=e−bt​
    ​

    where ​b>a>0.


    Let ​M(t)​ be the combined mass (in ​kg​) of the samples after ​t​ years.

    1. Show that ​M(t)2−M(2t)=2⋅A(t)⋅B(t).

      [3]

    After ​100​ years, the combined mass remaining is ​800g. After ​200​ years, the combined mass is ​400g.

    1. Using the identity in part (a), show that ​B(100)=25⋅A(100)3​​

      [3]
    2. Hence find ​A(100).

      [6]
    3. Find the exact value of ​a, giving your answer in the form ​klnp−lnq​, where ​p,p,k∈Z.

      [3]

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