2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/223466We construct several difference families on cyclic groups of orders 47 and 97, and use them to construct skew-Hadamard matrices of orders 188 and 388. Such difference families and matrices are constructed here for the first time. The matrices are constructed by using the Goethals-Seidel array.7 pages, no figures, a paragraph added to the introduction, two misprints corrected. To appear in the International Mathematical ForumCombinatorics05B20; 05B30text