Effective half-life multiple exponential decay

[NanakiXIII]

Say we have a decay of the form

A e^{-a x} + B e^{-b x}.

I haven't had much luck trying to calculate the half-life of such a decay (I'm not sure it's possible, analytically), i.e. solve

A e^{-a x} + B e^{-b x} = \frac{A+B}{2}.

However, if that's not possible, I'm wondering whether there might still be a way to prove that if the above decay has half-life t, then a decay given by

A e^{-a c x} + B e^{-b c x}

has half-life \frac{t}{c}. This seems to be true empirically and would make sense, I think. Does anyone have an idea how one might prove it to be true, though?

[Jmf]

Not really a 'proof', but I think it's rather obvious:

If f(t) is a function, then f(ct) is that function scaled to 1/c of it's size in the t-direction.

So if we plot the graph of these two functions, the point at which f(ct)=f(0)/2 will clearly be at 1/c times the point where f(t) = f(0)/2.



EDIT: I just read the above back to myself and it sounded a bit patronizing to say this was 'obvious' - sorry.

By way of a proof I suppose you could say let

f(x) = A e^{-a x} + B e^{-b x}.

and let

g(x) = A e^{-a c x} + B e^{-b c x}.

then:

g(x) = f(cx)

or equivalently:

g(\frac{y}{c}) = f(y)

So then if 't' is our half life of f, i.e. when:

f(t) = \frac{f(0)}{2}

then:

f(t) = g(\frac{t}{c})