The activity of a radioactive sample is A1 at time t1 and A2 at time t2. If τ is average life of sample then the number of nuclei decayed in time (t2–t1) is
(A1−A2)τ
Let N0 bet the initial number of nuclei, then
N1=N0e−λt1
and N2=N0e−λt2
∴ Number of nuclei decayed = N1−N2=N0(e−λt1−e−λt2)
=A0λ(e−λt1−e−λt2)=A1−A2λ=(A1−A2)τ.