y dx - x dy = (x² + y²) dx
y - x•dy/dx = x² + y²
- x•dy/dx = x² + y² - y
dy/dx = - x - y²/x + y/x
(y = - xu,y' = - u - xu')
- u - xu' = - x - xu² - u
- xu' = -x - xu²
du/(u² + 1) = dx
arctan(u) = x + C
u = tan(x + C)
- y/x = tan(x + C)
y = - xtan(x + C)