look Uzalfa,it’s really simple !

In series circuit there’s only one & simple path for current to flow so , the equation of series circuit is also simple R=R1+R2+R3 AND SO ON…so u c the equation for series circuit is so simple! …..but in PARALLEL circuit u know that there are wires which r type of dividing the circuit in boxes….i mean it looks that way so, also in equation of parallel circuit u have u have to divide he resistances with 1 …liKe this 1/RT=1/R1+1/R2 & SO ON…..I HOPE IT HELPS ðŸ™‚