Though a friar monk, I also tutored the sons of a wealthy merchant in Venice and gained a strong understanding of commercial arithmetic.
In 1494, I published Summa de arithmetica, geometrica, proportioni et proportionalita, which replaced Fibonacci's Liber abaci as the leading algebra text (only one?) at that time.
My Summa included an extensive documentation of double-entry bookkeeping techniques and the arithmetic of finding square roots.
Through my writings, I added to the Italian's standard use of the letters p and m for addition and subtraction, by introducing co, ce, and ae for the cosa (the unknown), censo (square of the unknown), and aequalis (equal to) respectively.
My last book was De divina proportione, an exploration of polygons, polyhedra, and the "golden section."
Luca Pacioli (1445-1514)