No. Of possible triangles If there are n line segments in a plane of lengths (1,2,3,....,N) then find the no.  of triangles that can be formed from these.
Attempt: let the sides be X,Y,Z such that X < Y< Z then the cardinality of required set boils down to finding out pairs such that X + Y > Z other two inequalities are automatically satisfied. Then I drew a table for values of X,Y and Z such that the above conditions hold.And now the solution becomes clumsy and lengthy. Is there a better way? And even in this approach for Z = n , I have (n-2)*(n-3)/2 solutions. And now this n can be anything from 4 to N. So I summed this again. But the problem is I am missing many things answer is different for odd and even values of N. What's wrong with my solution. It doesn't match with the correct answer. 

If $X<Y<Z$ and $X+Y>Z$,
$$Z+1\le X+Y\le Y-1+Y$$
and hence 
$$Y\ge \frac{Z}{2}+1$$
The number of triangles is
\begin{align}
\sum_{Z=4}^N\sum_{Y=\lfloor \frac{Z}{2}\rfloor+1}^{Z-1}\sum_{X=Z-Y+1}^{Y-1}1&=\sum_{Z=4}^N\sum_{Y=\lfloor \frac{Z}{2}\rfloor+1}^{Z-1}(Y-1-Z+Y)\\
&=\sum_{Z=4}^N\sum_{Y=\lfloor \frac{Z}{2}\rfloor+1}^{Z-1}(2Y-Z-1)\\
\end{align}
Note that 
\begin{align}
\sum_{Y=a}^{Z-1}(2Y-Z-1)&=2\left(\frac{Z-a}{2}\right)(a+Z-1)-(Z-a)(Z+1)\\
&=(Z-a)(a-2)\\
\end{align}
So, the number of triangles is
\begin{align}
\sum_{Z=4}^N(Z-\lfloor \frac{Z}{2}\rfloor-1)(\lfloor \frac{Z}{2}\rfloor+1-2)=\sum_{Z=4}^N(Z-\lfloor \frac{Z}{2}\rfloor-1)(\lfloor \frac{Z}{2}\rfloor-1)\\
\end{align}
If $N$ is even,
\begin{align}
&\;\sum_{Z=4}^N(Z-\lfloor \frac{Z}{2}\rfloor-1)(\lfloor \frac{Z}{2}\rfloor-1)\\
=&\;\sum_{k=2}^{N/2}(2k-\lfloor \frac{2k}{2}\rfloor-1)(\lfloor \frac{2k}{2}\rfloor-1)+\sum_{k=2}^{(N-2)/2}(2k+1-\lfloor \frac{2k+1}{2}\rfloor-1)(\lfloor \frac{2k+1}{2}\rfloor-1)\\
=&\;\sum_{k=2}^{N/2}(k-1)(k-1)+\sum_{k=2}^{(N-2)/2}k(k-1)\\
=&\;\sum_{i=1}^{(N-2)/2}i^2+\sum_{i=1}^{(N-4)/2}i^2+\sum_{i=1}^{(N-4)/2}i\\
=&\;2\left(\frac{1}{6}\right)\left(\frac{N-4}{2}\right)\left(\frac{N-4}{2}+1\right)(N-4+1)+\left(\frac{N-2}{2}\right)^2\\
&\qquad+\frac{1}{2}\left(\frac{N-4}{2}\right)\left(\frac{N-4}{2}+1\right)\\
=&\;\frac{1}{12}(N-2)(N-3)(N-4)+\frac{1}{4}(N-2)^2+\frac{1}{8}(N-2)(N-4)\\
=&\;\frac{1}{24}(N-2)[2(N-3)(N-4)+6(N-2)+3(N-4)]\\
=&\;\frac{1}{24}N(N-2)(2N-5)
\end{align}
If $N$ is odd, $N-1$ is even. So,
\begin{align}
&\;\sum_{Z=4}^N(Z-\lfloor \frac{Z}{2}\rfloor-1)(\lfloor \frac{Z}{2}\rfloor-1)\\
=&\;\frac{1}{24}(N-1)(N-1-2)[2(N-1)-5]+(N-\lfloor \frac{N}{2}\rfloor-1)(\lfloor \frac{N}{2}\rfloor-1)\\
=&\;\frac{1}{24}(N-1)(N-3)(2N-7)+(N-\frac{N-1}{2}-1)(\frac{N-1}{2}-1)\\
=&\;\frac{1}{24}(N-1)(N-3)(2N-7)+\frac{1}{4}(N-1)(N-3)\\
=&\;\frac{1}{24}(N-1)(N-3)(2N-7+6)\\
=&\;\frac{1}{24}(N-1)(N-3)(2N-1)
\end{align}
