Use Euler’s Formula to find the number of vertices in a polyhedron with four triangular faces.

Euler formula:
[tex]V-E+F=2[/tex]
Since [tex]F=4[/tex] then 
[tex]V-E+4=2\\V-E=-2\\V=E-2[/tex] and [tex]E=8[/tex] (check the figure)
So:
[tex]V=8-4=4[/tex] is the number of vertices.