Introduction to Functional equations and Ulam stability theory.- Stability and instability of multiplicative inverse type tredecic and quottuordecic functional equations in non-Archimedean spaces.- Estimation of inexact multiplicative inverse type quindecic and sexdecic functional equations in Felbin's type fuzzy normed spaces.- Classical approximations of multiplicative inverse type septendecic and octadecic functional equations in quasi-β-normed spaces.- Ulam stabilities of multiplicative inverse type novemdecic and vigintic functional equations in intuitionistic fuzzy normed spaces.- Solution to the Ulam stability problem of multiplicative inverse type unvigintic and duovigintic functional equations in paranormed spaces.- Inexact solution of multiplicative inverse type trevigintic and quottuorvigintic functional equations in matrix normed spaces.