This differential equation is separable:
(1 + y²) dx + (1 + x²) dy = 0
(1 + y²) dx = - (1 + x²) dy
dy/(1 + y²) = -dx/(1 + x²)
Integrating both sides gives
arctan(y) = -arctan(x) + C
and solving for y gives (over an appropriate domain)
y = tan(C - arctan(x))
(the domain being -1 ≤ y ≤ 1).
