In Exercises 7-11, find the general solution. In some cases, it is helpful to carry
out the indicated differentiation, in others it is not.
7. $\frac{d}{dx}( (h + kx)\frac{dv}{dx}) = 0$ ($h$, $k$ are constants)
8. $(e^x \phi)' + \lambda^2 e^x \phi = 0$
9. $\frac{d}{dx} (x^3 \frac{du}{dx}) = 0$
10. $r^2 \frac{d^2 u}{dr^2} + r \frac{du}{dr} + \lambda^2 u = 0$
11. $\frac{1}{r} \frac{d}{dr} (r \frac{du}{dr}) = 0$
12. Compare and contrast the form of the solutions of these three differential
