Question

If you draw all of the diagonal from a single vertex of a convex polygon with 8 sides,how many triangles are found

Answer 1

The number of triangles in a polygon is found by taking the number of sides and subtracting 2.  8 - 2 = 6 triangles.

Answer 2

Remark
There is a formula that describes this event. In a convex polygon with n sides, you can create n - 2 triangles.

Without loss of generality, you could call this a regular octagon. It will not matter that it is regular; it will still be convex. See the diagram below. There are 6 such triangles.