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Who was the first person to use calculus in a practical way?
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That would be the Italian mathematician Luca Pacioli. He published the book Summa de Arithmetica, Geometria, Proportioni et Proportionalita in 1509, which was the first book on algebra. In this book, he introduced the concepts of derivatives and integrals, and was the first to use them in a practical way. He also invented the first double entry accounting system.