Connecting every two adjacent corners of a decagon requires five diagonals, with a Pentagon in the middle. The two diagonals turn the Pentagon into three triangles, so the total number of * * * is seven;

Second, (n-2)

An N-sided polygon has n vertices, from which at most (n-3) diagonals can be drawn, and each diagonal can have two triangles, so the polygon is always divided into (n-2) triangles.

Three. Article 54

According to two, we can get n= 1 1, so (n+ 1)= 12. Because the diagonal number of an N-polygon can be obtained by mathematical induction: n*(n-3)/2. So the answer is 54;

4. An N-polygon has n vertices, and at most (n-3) diagonals can be drawn from a certain vertex. The diagonal number of an N-polygon can be obtained by mathematical induction: n*(n-3)/2.

Verb (abbreviation of verb) 1 and 2; 2, 5, 3 more; 3, 9, 4 more; 4. The diagonal number of the N-polygon can be obtained by mathematical induction: n*(n-3)/2. When n=n+ 1 in the formula, the number of strips =(n+ 1)*(n-2)/2. (n+ 1)*(n-2)/2 minus n*(n-3)/2 equals (n- 1).