How many triangles are formed by the diagonal from one vertex in quadrilateral, hexagon, octagon, and decagon?!

Quadrilateral: two (you can only trace one diagonal and it forms two triangles)

Hexagon: four (you can trace thre diagonals and four triangles are formed)

Octagon: six (you can trace five diagonals and six triangles are formed)

Degagon: eight (you can trace seven diagonals and eight triangles are formed)

It is always true that the number of triangles you can form from a vertex in a convex polygon is the number of sides minus 2.