b
\(R = R_0e^{-\lambda\,t_2}\)  (ઍક્ટિવિટીના નિયમ મુજબ)

\(\therefore\)\({R_1} = {R_0}{e^{ - \lambda {t_1}}}\)  અને \({{\text{R}}_{\text{2}}} = {R_0}{e^{ - \lambda {t_2}}}\)

\(\therefore\)\(\frac{{{R_1}}}{{{R_2}}} = \frac{{{R_0}{e^{ - \lambda {t_1}}}}}{{{R_0}{e^{ - \lambda {t_2}}}}} \)

\(= {e^{ - \lambda {t_1}}}{e^{ - \lambda {t_2}}} = {e^{ - \lambda ({t_1} - {t_2})}}\)

\(\therefore\) \({R_1} = {R_2}{e^{ - \lambda ({t_1} - {t_2})}}\)