Rotation curves

Astaroth.

Hello.
I need help with a truncation of the surface density of a thin exponential disk.
To calculate the speed.
At [itex] \sigma (r)) = \sigma_{0} \exp(-\frac{r}{L}) [/itex]

Help to calculate the formula for [itex] \sigma (r) = \sigma (Rt)*(1+(Rt-r)/SCL)= \sigma_{0} * \exp(-\frac{Rt}{L})*(1+(Rt-r)/SCL) [/itex] and [itex] \sigma (r) = \sigma (Rt) = \sigma_{0} * \exp(-\frac{Rt}{L}) [/itex]