怎麽會這樣呢？不至於吧

2.
|x(x-a)| = x(x-a) when x >=a
|x(x-a)| = x(a-x) when x
3.
Let b = a - 1. Then 0 a1 = a, a2 = a - 1 = b,
a3 = b - 1 = - (1-b), a4 = (1-b) - 1 = -b
a5 = b - 1 = - (1-b), a6 = (1-b) - 1 = -b
......

5.
(x - 2y)^2 + (y + 1)^2 = 1 + 4
There are a total of 8 defferent pairs of integer solutions by considering the signs and symmetries.