20 чисел
21,23,25,27,29,41,43,45,47,49,61,63,65,67,69,81,83,85,87,8920 чисел
21,23,25,27,29,41,43,45,47,49,61,63,65,67,69,81,83,85,87,89
(2a + 1)² - (a - 9)² = (2a + 1 + a - 9)(2a + 1 - a + 9) = (3a - 8)(a + 10)
Ответ:
x=-6