Black Hole Interior Mass Formula
We argue by explicit computations that, although the area product, horizon radii product, entropy product and \emph {irreducible mass product} of the event horizon and Cauchy horizon are universal, the \emph{surface gravity product}, \emph{surface temperature product} and \emph{Komar energy product} of the said horizons do not seem to be universal for Kerr-Newman (KN) black hole space-time. We show the black hole mass formula on the \emph{Cauchy horizon} following the seminal work by Smarr\cite{smarr} for the outer horizon. We also prescribed the \emph{four} laws of black hole mechanics for the \emph{inner horizon}. New definition of the extremal limit of a black hole is discussed.