... (Afg).3. Stability of the equation (Tfg) In this section, we investigate the stability of the trigonometric functional equations (Tfg)related to the sine equation (S) and the cosine equation (A).Theorem ... constant, a mapping ϕ : G→R.2. Stability of the equation (Tgf) In this section, we investigate the stability of the t rigonometric functional equation (Tgf)as related to the cosine-, the ... The stability of the cosine equation,” Proceedings of the American MathematicalSociety, vol. 80, no. 3, pp. 411–416, 1980.[4] P. W. Cholewa, The stability of the sine equation,” Proceedings...