The function is given by the table.

f: N\{1,2}-> N_{o}

__n_| 3 | 4 | 5 | 6 | 07 | 08 | 09 | 10 |...

f(n) | 0 | 2 | 5 | 9 | 14 | 20 | 27 | 35 | ...

1) which functional equation characterizes this function?

2) solve this difference equation.

3) express this function in an explicit form

4) what is this function of the planimetry?

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my opinions> It is ok 3) and 4). this function is the number of diagonals of a polygon with n sides, and $\displaystyle f(n)=\frac{n\cdot(n-3)}{2}$

please help me about 1) and 2). how do I find the difference equation and functional equetion?

thank you!