I've been working on a problem where I have to find the geodesics for a given Riemannian Manifold. To present my doubt, I tried to find a simpler example that would demonstrate my uncertainty but the one I found, and shall present bellow, has actually a simplification that my problem doesn't, so please ignore that simplification (I shall indicate it when it comes up).
Given a Riemannian manifold with metric $$\tag{1} ds^{2}=\frac{dr{}^{2}}{a+r^{2}}+r^{2}\left(d\theta{}^{2}+\sin^{2}(\theta)\,d\phi^{2}\right),$$ consider a curve $c(\lambda)=(r(\lambda),\phi(\lambda),\pi/2)$ whose tangent vector is $c'(\lambda)=(r'(\lambda),\phi'(\lambda),0)$. The geodesic equations (if I didn't mess up) are given by: \begin{cases} \tag{2} r''-\frac{r'^{2}}{2(a+r)}-r(a+r)\phi'^{2}=0,\\ \phi''+2\frac{r'}{r}\phi'=0. \end{cases}
The second equation of Eq.$(2)$ can be integrated, such that: $$\tag{3} \phi'(\lambda)=\frac{C}{r^{2}(\lambda)},$$ where $C$ is a constant of integration.
Now I introduce a new equation: $$\tag{4}r'^{2}\left(\frac{1}{a+r^{2}}\right)+\phi'^{2}r^{2}=1,$$ which is basically impose unit speed. Substituting Eq.$(3)$ in Eq.$(4)$ we have that: $$\tag{5} r'=\sqrt{(a+r^{2})\left(1-\frac{C^{2}}{r^{2}}\right)}.$$ (Here is the difference from my case since Eq.$(5)$ can be integrated analytically and in my case, the congener equation can't).
Dividing Eq.$(3)$ by Eq.$(5)$ we have that $$\tag{6} \frac{d\phi}{dr}=\frac{C}{r^{2}(\lambda)}\frac{1}{\sqrt{(a+r^{2})\left(1-\frac{C^{2}}{r^{2}}\right)}}.$$
So my question is:Does Eq.$(6)$ allow me to write the curve $c$ as $c(r)=(r,\phi(r))$, where $\phi(r)$ is given by $(6)$ and $c$ is a geodesic?
Note: This treatment is based on the Clairaut parametrization but I'm not sure if I can do it in this kind of problem where the $g_{rr}$ component of the metric varies...
There is something wrong with your formula for the metric. The expression $ds^{2}=\frac{dr{}^{2}}{a+r^{2}}+r^{2}\left(d\theta{}^{2}+\sin^{2}(\theta)\right)\,d\phi^{2}$ does not define a metric because $d\theta^2$ and $d\phi^2$ are being multiplied. Are some of the parentheses off?