When a regular polygon with \( n \) sides is decomposed into triangles using the same vertex, the number of triangles formed is given by the formula \( n - 2 \). This is because you can choose a vertex and draw diagonals to the other non-adjacent vertices, creating a number of triangles equal to the total number of sides minus 2.
Therefore, the correct response is:
When a regular polygon is decomposed, \( n - 2 \) triangles are formed, where \( n \) is the number of sides of the regular polygon. There are two fewer triangles than the number of the sides of the regular polygon.