public static SquareMatrix <int> Sum(this Matrix <int> matrix1, DiagonalMatrix <int> matrix2) { matrix1.CheckMatrix(); matrix2.CheckMatrix(); IsMatrixEquals(matrix1.Number, matrix2.Number); this.Number = matrix1.Number; this.resultMatrix = new int[Number, Number]; for (int i = 0; i < Number; i++) { for (int j = 0; j < Number; j++) { resultMatrix[i, j] = matrix1[i, j] + matrix2[i, j]; } } return(resultMatrix); }