The activity of a sample of radioactive material is `A_1` at time `t_(1)` and `A_(2)` at time `t_(2)(t_(2) le t_(1))` . Obtain an expression for its mean life.

Explanation

`A_(1)=A_(0) e^(- lambda t_(1))` and `A_(2) =A_(0) e^(- lambda t_(2))`
`:. (A_(1))/(A_(2))=e^(lambda^((t_(2) -t_(1))`
`rArr lambda(t_(2) -t_(1))=In|A_(1)/(A_2)| rArr T=(1)/(lambda)=(t_(2)-t_(1))/(In |(A_(1))/(A_(2))|`.