### Differential Equations and Normal Line

Question 1. 1) Find a vector normal to the surface z + 2xy = x2 + y2 at the point (1,1,0). 2) Determine if there are separable differential equations among the following ones and explain: a) dy/dx=sin(xy), b) dy/dx = (xy)/(X+y) c) dr/d(theta) = (r^2+1)cos(theta) 3) Find the general solution of the differential