# Finding number of Vertices and Edges of a Graph via Euler's Formula

Let $G$ be a simple $4$-regular connected graph, and suppose that $G$ is planar and has $10$ faces. (A graph is $4$-regular if all of its vertices have degree $4$.)

• Determine the number of edges of $G$.
• Determine the number of vertices of $G$.

I know about the Euler's formula $n-m+f = 2$.

Therefore with $f = 10$ it can be rearranged to: $8= -n + m$.

However I am stuck as to continue to gain exact values for $n$ and $m$.

## 1 Answer

You have to use the fact that the graph is $4$ - regular, so all degrees of vertices are $4$.

Then, remember that the sum of all vertex degrees is closely connected (i.e. there exists a formula to connect them) to the number of edges.

Now, the sum of vertex degrees is simply $4\cdot n$, since you have $n$ vertices and each has degree $4$. Plugging this into the formula connecting vertex degrees with $m$ will give you one more equation for $m$ and $n$ next to the one you already have. Then, simply solve the system of equations.