$${\frac{{\mathtt{1}}}{{\mathtt{RP}}}} = {\frac{{\mathtt{1}}}{{\mathtt{R1}}}}{\mathtt{\,\small\textbf+\,}}{\frac{{\mathtt{1}}}{{\mathtt{R2}}}}$$
Multiply each term of the equation by the common denominator: RP · R1· R2 and simplify each term:
R1 · R2 = RP · R2 + R1· RP
Factor out the RP from the right side:
R1 · R2 = RP · ( R2 + R1 )
Divide both sides by ( R2 + R1 ):
RP = R1 · R2 / ( R2 + R1 )
