Department of Mathematics, Kangnam University, Yongin, Gyeonggi 446-702, South Korea
Copyright © 2010 Gwang Hui Kim. This is an open access article distributed under the
Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
We will investigate the superstability of the (hyperbolic) trigonometric functional equation from the following functional equations: and, which can be considered the mixed functional equations of the sine function and
cosine function, the hyperbolic sine function and hyperbolic cosine function, and
the exponential functions, respectively.