DP/dt = m(a0)[exp(-z1)t] - (z2/z1)P
Solve this differential equation with a = ao at t=0 and a=a at t=t to show that:
P = [mz1(a)] / [z2 - z12] + [mz1(ao) / (z12- z2)](a/ao)^(z2/z12)
Where the last term in this equation is a/ao "raised to the power of" z2/z12
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