# Find the number of triangles in plane

Let $$S$$ be a set of $$n$$ points in the plane in general position. Each 3 points of S span a triangle. Total number of triangles spanned by S:

$$\binom{n}{3}=\frac{n(n-1)(n-2)}{6}=\frac{1}{6} n^3-O(n^2 )$$

We know that Every set of $$n$$ points in general position in the plane can be partitioned into $$6$$ parts of almost equal size by $$3$$ concurrent lines (at least $$\frac{n}{6}-c$$(where $$c$$ is constant) points in each part).

Let $$x$$ be the point of intersection of the $$3$$ lines.

I need to prove that x is contained in at least $$\frac{n^3}{27}-O(n^2 )$$ triangles.

My approach:

•Every triangle of the form ACE contains $$x$$, the number of the triangles of this form $$\frac{n^3}{216}.$$

•Every triangle of the form BDF contains x, the number of the triangles of this form $$\frac{n^3}{216}.$$

•For every edge of the form AD, either adding any point of E makes a triangle that contains x, or adding any point of C, the number of the triangles of this form $$\frac{n^2}{36}(\frac{n}{6}+\frac{n}{6})=\frac{n^3}{108}.$$

How to proceed after then? I have stucked at this stage.

• For ACE pattern you have $n^3/216$ triangles, not $n^3/6$. Totally you already got $2\cdot(n^3/216)+3\cdot (n^3/108)=n^3/27$ triangles, as needed. Commented Aug 9 at 2:57
• @FedorPetrov yes, you are right, would you put as answer? I will accept it. Commented Aug 9 at 2:59

Your argument already gives $$2\cdot\frac{(n-6c)^3}{216}+3\cdot \frac{(n-6c)^3}{108}=\frac{(n-6c)^3}{27}$$ triangles.