I am trying to understand the derivation of a logarithmic amplifier as shown in the figure attached. The diode has a non linear characteristics represented by V=ClnI,where C is a constant. Then since the current through the feed back loop is the same as the current through the input resistance and the potential difference across the diode is -Vout,we have Vout=-Cln(Vin/R)=Kln(Vin) where R is input resistance. What I cant understand is the step in bold,what mathematical rule is used between the two equations? Thank you in advanced .