V  = V1+V2+V3+V4  and is known as Kirchoff's voltage rule (or law)

V  =  Is (R1 + R2 + R3 + R4)

Rs    = R1 + R2 + R3 + R4

Rs     = R1 + R2 + R3 + R4 + ... RN

Ip   =   I1 + I2 + I3 + I4

Ip   =   V/R1  + V/R2  + V/R3 + V/R4

             Ip   =   V ( 1/R1  + 1/R2  + 1/R3  + 1/R4)

Ip   =   V/Rp

V/Rp  =   V ( 1/R1  + 1/R2  + 1/R3  + 1/R4)

1/Rp  =    1/R1  + 1/R2  + 1/R3  + 1/R4

1/Rp  =    1/R1  + 1/R2  + 1/R3  + 1/R4  +  ... 1/RN