Differential Equations
posted by Quinn .
Model radioactive decay using the notation
t = time (independent variable)
r(t) = amount of particular radioactive isotope present at time t (dependent variable)
λ = decay rate (parameter)
Note that the minus sign is used so that λ > 0
a) Using this notation, write a model for the decay of a particular radioactive isotope.
b) If the amount of the isotope present at t = 0 is r0, state the corresponding initialvalue problem for the model in part (a).

the rate is proportional to r(t) times a constant
dr/dt=r(t)* constant where the constant is negative
and the solution to this first order diff equation is of the form
r(t)=K*e^( Î»)t + C
b. ro=K+C
and at t=inf, r(inf)=0
which implies C is zero, so k=ro.
r(t)=ro*e^ Î» t 
a) dr(t)/dt = λr(t)
=>∫[ dr(t)/r(t) ] = ∫[ λt ]
=> ln r = λt + C
=> r(t) = e^(λt + C) = e ^ (C  λt)
=> r(t) = e^C * e ^ (λt)
let C = e^C
=> r(t) = Ce^(λt)
b) r(0) = r_0
=>r(0) = r_0 = Ce^(λ(0))
=>r_0 = C
=>r(t) = r_0 * e^(λt)