一
(a + 2b - 1)(a - 2b - 1) = [(a - 1) + 2b][(a - 1) - 2b]
二
(x + 1/x)^2 = x^2 + (1/x)^2 + 2 = 25
(x - 1/x)^2 = x^2 + (1/x)^2 - 2 = 21
x - 1/x = ±√21