Triangle inequality
For any triangle, the sum of the lengths of any two sides must be greater than or equal to the length of the remaining side.
When entering
LocusEquation[a+b==c,A] this statement can be doubted. But GeoGebra returns the line BC which means that point A must lie on BC in order to satisfy a+b=c. This actually means that the triangle inequality is true, because in GeoGebra's answer the triangle must be degenerate.