book about solving quadratic equations principial mathematical text-book of European Universities until 16th century al-ğabr ("forcing", "restoring") operation :: moving a deficient quantity from one side of the equation to the other side al-Muqabala ("balancing" or "corresponding") operation :: subtraction of the same positive quantity from both sides presented in ordinary text Al-kitāb al-mukhtaṣar fī ḥisāb al-ğabr wa’l-muqābala, The Compendious Book on Calculation by Completion and Balancing