v 11.13

d I_{f}^{st} = h f (\frac{d N_{2}}{d t})_{st} d z = \frac{h f B_{21}}{c} N_{2} I_{f} d z

${\mathrm{d}I_f^\text{st}=hf \bigg( \frac{\mathrm{d}N_2}{\mathrm{d}t} \bigg)_\text{st} \mathrm{d}z=\frac{h f B_{21}}{c} N_2 I_f \mathrm{d}z}$

Error