Prove that $a^2 + b^2 + c^2 \lt 2(ab + bc + ca)$ Given that $a, b$ and $c$ are the sides of a triangle.
How to prove that $a^2 + b^2 + c^2 \lt 2(ab + bc + ca)$?
Maybe any hint? Am I going to wrong direction?
$$2(ab + bc + ca)-a^2 + b^2 + c^2>0$$
$$2ab + 2bc + 2ca-a^2 + b^2 + c^2>0$$
$$2b(a+c) + 2ca-a^2 + b^2 + c^2>0$$
...?
 A: Note that three sides $a,b,c$ in a triangle satisfy
$$ a<b+c, b<a+c, c<a+b. $$
So one has
$$ a^2<a(b+c), b^2<b(a+c), c^2<c(a+b). $$
Adding these three inequalities gives
$$ a^2+b^2+c^2<2ab+2bc+2ca. $$
A: Note that we have $$a^{2}>(b-c)^{2}\;,b^{2}>(a-c)^{2}\;,c^{2}>(b-a)^{2}$$
As $a,b,c$ are sides of a trinagle. Adding these, we get $$a^{2}+b^{2}+c^{2}>2a^{2}+2b^{2}+2c^{2}-2ab-2bc-2ca$$
Which is equivalent to $$a^2+b^2+c^2<2ab+2bc+2ca$$
A: You can even prove more: if $a,b,c$ are the sides and $\alpha,\beta,\gamma$ the respectively opposite angles, summing up the three identities of the law of cosine, we get
$$
a^2+b^2+c^2=2ab\cos\gamma+2bc\cos\alpha+2ca\cos\beta
$$
which implies the statement, as all the cosines are less than $1$.
A: Alternatively, by cosine law
\begin{align*}
  b^2+c^2-a^2 &= 2bc\cos A \\
  c^2+a^2-b^2 &= 2ca\cos B \\
  a^2+b^2-c^2 &= 2ab\cos C \\
  a^2+b^2+c^2 &= 2(bc\cos A+ca\cos B+ab\cos C) \\
  & \le 2(bc+ca+ab)
\end{align*}
A: Let $a=y+z$, $b=x+z$ and $c=x+y$.
Hence, $x$, $y$ and $z$ are positives and
$$\sum_{cyc}(2ab-a^2=\sum_{cyc}(2(x+z)(y+z)-(y+z)^2)=$$
$$=\sum_{cyc}(2x^2+6xy-2x^2-2xy)=4(xy+xz+yz)>0$$
