(1+y²)dx + (1+x²)dy = 0

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).

because they could drop even lower..