dQ/dt = λ1P – λ2Q (1).

Similarly dR/dt = λ2Q – λ3R (2).

Substituting in (1) the value of P in terms of n,

The solution of this equation is of the form

By substitution it is found that a = λ1/(λ2 – λ1).

Since Q = 0 when t = 0, b = -λ12 – λ1).

Thus