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Algebra is the primary subject of Al-Jabr, a mathematical treatise written by the Persian mathematician Muhammad ibn Musa al-Khwarizmi in the 9th century. Al-Jabr, meaning "reunion of broken parts," introduced the concept of solving equations by manipulating unknown quantities represented by symbols. This groundbreaking work laid the foundation for modern algebra and revolutionized mathematics, making it a cornerstone of scientific and technological advancements.