Question:

Which must be true: I) There are $n\ge 3$ straight lines, no two parallel, no three concurrent. Points of intersection are joined to form fresh lines — number of new lines is $\frac{n(n-1)(n-2)(n-3)}{8}$. II) Vertices of $n$-sided polygon joined to form triangles — number of obtuse angled triangles is $n\cdot {}^{n/2-1}C_2$ if $n$ is even ($n>4$).