  Using the fixed point alternative theorem we establish the orthogonal
stability of quadratic functional equation of Pexider type
$f(x+y)+g(x-y)=h(x)+k(y)$, where $f, g, h, k$ are mappings from a symmetric
orthogonality space to a Banach space, by orthogonal additive mappings under a
necessary and sufficient condition on $f$.
