Clearly if $a$, $b$ and $c$ are the lengths of the sides of a triangle and the triangle is equilateral then
$a^2 + b^2 + c^2 = ab + bc + ca$.
Is the converse true, and if so can you prove it? That is if $a^2 + b^2 + c^2 = ab + bc + ca$ is the triangle with side lengths $a$, $b$ and $c$ necessarily equilateral?