Find the general solution of each PDE. The solution u is a function of the variables which appear, unless otherwise stated.
1. u_y = 2x
2. u_x = sin x + cos y
3. u_x = sin x + cos y, u = u(x, y, z)
4. u_yy = x^2y
5. u_xy = x - y
6. u_xxy = 0
7. u_xxyy = sin 2x
8. u_xzz = x - yz + y^3
9. u_xyzz = 0
10. u_xyyzz = xyz
11. u_y - 4u = 0, u = u(x, y)
12. u_x + 3u = e^x, u = u(x, y)