In the 18th-century, students would copy extracts from books and copies of carefully worked computations into their own copy-books that they could then use for study and reference.
Some of George Washington’s original copy or cypher-books are known and the authors work backwards to discover his sources. They show that George Washington learned decimals and some work with logarithms and trigonometry. They also show that he did not use this theory in his surveying problems, relying on measurement of accurate scale drawings rather than computations to find sides of triangles. In a nice turn of phrase, they say Washington “encountered” mathematics, setting aside the questions of how much he understood, let alone used.