Rav Hai Gaon ben Rav Sherira Gaon was made Rosh Yeshiva in his father's lifetime in year 4757 [(and in Yuchsin in the order of the Geonim, generation 7, he wrote 4758)] and he spread Torah in Israel more than all the Geonim. Those seeking Torah came to his light from east to west, and there was none like him before him among the Geonim, and he was the last of the Geonim. He was twenty-nine years old when he reigned [(and in Shalshelet HaKabbalah, letter 37, part 3, he wrote twenty-eight years old)] and he reigned forty years. [And in the responsa of the Maharshal, section 29, they ordained him for the Gaonut at twenty years old in year 4736.
Yuchsin wrote in the order of the Geonim, generation 7, that his generation was the eighth generation of the Gaonut. And in Seder HaKabbalah of the Rema he wrote that the days of his life were about ninety years. He passed away in year 4798 (but it requires examination, since according to all opinions he was not more than twenty-nine when he reigned and he reigned forty, how were his life ninety years?).
In the Mordechai on Shabbat it appears that Rav Hai was in Rome. He composed the book Musar HaSekhel, rulings and customs in proverbs and ethics and in rhyme from year 4722. And the book of Sales and Purchases, and Laws of Oaths, and a new method from many on the Blessing of Yaakov and the construction of the work of the Mishkan from year 4762, and Sefer HaPikkadon. His burial is in the Metivta of Machsaya (some wrote) and in Gedolei Yisrael he wrote that Rav Hai Gaon is buried near Mount Sinai.] In those days the wicked of Israel reported on Rav Sherira Gaon and Rav Hai his son, and the Ishmaelite king seized them and plundered all they had and left them no sustenance.
Rav Sherira Gaon was hung by his one hand when he was already one hundred years old. [And Shalshelet HaKabbalah, letter 37, part 3, wrote that he died around year 4780.]