Triangles and inequalities Let ABC be a triangle and let O be any point in space. How can I show that  $AB ^ 2 + BC ^ 2 + CA ^ 2 \leq 3 (OA ^ 2 + OB ^ 2 + OC ^ 2)$?
I know this prove by inner products, but is possible to show this by euclidean geometry?
 A: I'll use $P$ instead of $O$ through the course of this answer; $O$ is a little confusing because it usually denotes a centre.
Let $D$, $E$ and $F$ be midpoints of $BC$, $CA$, and $AB$ respectively; let $G$ be the intersection of $AD$, $BE$, and $CF$, i.e. the triangle's centroid. It is well known that $AG$ = $2GD$ (and similar for the other medians). By Apollonius's theorem, along with the fact that $AD = \frac 3 2 AG$, we note that
$$
\frac 9 2 AG^2 + \frac 1 2BC^2 = AB^2 + AC^2;
$$
summing the symmetric equations for the other medians we have
$$
\frac 9 2 (AG^2 + BG^2 + CG^2) = \frac 3 2 (AB^2 + BC^2 + CA^2).
$$
In other words, $G$ makes the inequality tight. Thus we are clearly motivated to try and do something with $G$.
Consider the following diagram.

By Stewart's theorem (a slight generalisation of Apollonius's theorem, and only a few steps removed from direct Pythagoras bashing) we obtain the result that
$$
\frac 2 3 PD^2 + \frac 1 3 PA^2 = PG^2 + \frac 2 9 AD^2 = PG^2 + \frac 1 2 AG^2.
$$
To get rid of the $PD^2$, we use Apollonius on $\bigtriangleup \!\! PCB$:
$$
2PD^2 + \frac 1 2 BC^2 = PB^2 + PC^2.
$$
Multiplying the former expression by $3$ and subtracting the latter we obtain
$$
PA^2 - \frac 1 2 BC^2 = 3PG^2 + \frac 3 2 AG^2 - PB^2 - PC^2.
$$
Summing cyclically for the corresponding expressions oriented about medians $BE$ and $CF$, and substituting in our previous expression for the sum of square distances at the centroid $G$, we obtain
$$
3(PA^2 + PB^2 + PC^2) = 9PG^2 + AB^2 + BC^2 + CA^2,
$$
so we are done by non-negativity of squares.
A: If we find such point $D$ that $$3(OA^2+OB^2+OC^2)=3(DA^2+DB^2+DC^2+3OD^2)\tag{1}$$ then we will only have to show that $$AB^2+BC^2+CA^2\le 3(DA^2+DB^2+DC^2)\tag{2}$$
As I'm too lazy to prove this geometrically about sufficent choice of $D$ as median intersection point (i.e. centroid) of $\triangle ABD$, let's put the entire thing into Cartesian coordinates, letting $A(a,b),\,$ $B(c,d),\,$ $C(u,v),\,$ $O(x,y),\,$ $D(0,0)$, then $$OA^2+OB^2+OC^2-(DA^2+DB^2+DC^2+3OD^2)=\\
-2 (a x + b y + c x + d y + u x + v y)$$
so we select $D$ and move the origin into it such a way, that 
$$a+c+u=0,\,b+d+v=0$$
And now we have to prove $(2)$ with this choice of $D$, as we have $(1)$, but in this case the equality holds $$AB^2+BC^2+CA^2=3(DA^2+DB^2+DC^2)\tag{3}$$
so we're done.
Update: also the equalities $(1)$ and $(3)$ are given on the wiki (Centroid) with $O$ being $P$ and $D$ being $G$ there.
