In this paper, we prove the generalized Hyers–Ulam stability of a quadratic and quartic functional equation in intuitionistic fuzzy Banach spaces.

The goal of this paper is to investigate the Generalized Hyers - Ulam Rassias (HUR) stability generalquartic functional equation (GQFE) fq(kr1 + (k -1)r2) = 2k4fq(r1) 2(k -1)4fq(r2) +6k2(k -1)2[fq(r1 r2) fq(r1 r2)] 12k2(k 1)2[fq(r1) fq(r2)] in fuzzy normed spaces (F.N. spaces). proved by using direct method. sense and Ulam- Gavruta also studied.

In this paper, we investigate the Hyers-Ulam stability of the orthogonally  cubic equation and  Pexiderized cubic equation [f(kx+y)+f(kx-y)=g(x+y)+g(x-y)+frac{2}{k}g(kx)-2g(x),]in multi-normed spaces by the direct method and the fixed point method. Moreover, we prove the Hyers-Ulam stability of the  $2$-variables cubic  equation [ f(2x+y,2z+t)+f(2x-y,2z-t) =2...

We will show the general solution of the functional equation f(x + ay) + f(x− ay) + 2(a − 1)f(x) = af(x + y) + af(x− y) + 2a(a − 1)f(y) and investigate the Hyers-Ulam stability of the quartic set-valued functional equation.