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Brahmagupta (598-670) writes Khandakhadyaka which solves quadratic equations and allows for the possibility of negative solutions.
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1136 Abraham bar Hiyya Ha-Nasi writes the work Hibbur ha-Meshihah ve-ha-Tishboret, translated in 1145 into Latin as Liber embadorum, which presents the first complete solution to the quadratic equation.
1484 Nicolas Chuquet (1445-1500) writes Triparty en la sciences des nombres. The fourth part of which contains the "Regle des premiers," or the rule of the unknown, what we would today call an algebra. He introduced an exponential notation, allowing positive, negative, and zero powers. In solving general equations he showed that some equations lead to imaginary solutions, but dismisses them ("Tel nombre est ineperible").