dp/p+dv/v=(1-r)dv/v
its next step is
dp/p+rdv/v=0
can you please tell me urgently that how zero come in rhs??

Simply subtract (1-r)dv/v from both sides of the equation:

\frac {dp}p + \frac {dv} v -(1-r) \frac {dv}v = (1-r)\frac {dv}v - (1-r) \frac {dv} v

\frac {dp} p + \frac {dv} v -\frac {dv}v + r \frac {dv} v = 0

\frac {dp} p + r \frac {dv} v = 0