The activity of a sample of radioactive material is `R_(1)` at time `t_(1)andR_(2)"at time"t_(2)(t_(2)gtt_(1))`. Its mean life is T. Then,

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))|`.