B1 = 5; b9 = 25
b9 = b1 · q^8
25 = 5 · q^8
q^8 = 5
q = 5^(1/8)
Образуем геометрическую прогрессию
b1 = 5;
b2 = 5^(1 + 1/8) = 5^(9/8)
b3 = 5^(9/8 + 1/8) = 5^(10/8) = 5^(5/4)
b4 = 5^ (10/8 + 1/8) = 5^(11/8)
b5 = 5^(11/8 + 1/8) = 5^(12/8) = 5^(3/2)
b6 = 5^(12/8 + 1/8) = 5^(13/8)
b7 = 5^ (13/8 + 1/8) = 5^(14/8) = 5^(7/4)
b8 = 5^(14/8 + 1/8) = 5^(15/8)
b9 = 5^(15/8 + 1/8) = 5^(16/8) = 5² = 25
Задача решена