What is a chromatic number of a graph

Question

Discussion:

Q: Show that the chromatic number of G_1 + G_2 isX(G_1) + X(G_2) for any two graphs G_1 and G_2.

Where X(G)is the Chromatic Number.
defn: X(G) a proper colouring or simply a colouring of the vertices of G is an assignment of colours to the vertices in such a way that adjacent vertices have distinct colours; X(G) is then the minimal number of colours in a (vertex) colouring of G. Thus, for example, X(C_(2k))=2 and X(C_(2k+1))=3

Chromatic number:

The chromatic number of a graph is the minimum number of colours required to colour it.

https://mathworld.wolfram.com/ChromaticNumber.html
https://planetmath.org/encyclopedia/ChromaticNumber.html