calculus

The rate of decay in the mass, M, of a radioactive substance is given by the differential equation dM dt equals negative 1 times k times M, where k is a positive constant. If the initial mass was 100g, then find the expression for the mass, M, at any time t.

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  1. dM/dt = -k M
    separate variables, M left, t right
    dM/M = -k dt
    integrate
    ln M = -k t + c

    note e^log a = a
    ao
    e^ln M = M = e^(-kt+c) = e^-kt e^c
    or since e^c is some constant call it C
    M = C e^-kt
    note that when t = 0
    e^-kt = 1
    so
    C = initial amount
    so here
    M = 100 e^-kt

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  2. Damon is right

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