 Solution

If a bilinear map is continuous, continuity at $(0,0)$ gives $\delta >0$ such that $\Vert F(x,y)\Vert \le 1$ whenever $\Vert x\Vert ,\Vert y\Vert <\delta$. For nonzero $x,y$, apply this to $\delta x/(2\Vert x\Vert )$ and $\delta y/(2\Vert y\Vert )$. Bilinearity givesIf either input is zero, the inequality follows from bilinearity directly.

Conversely, suppose the product bound holds with $C\ge 0$. At a fixed point $(x,y)$,Its norm is at most $C(\Vert h\Vert \Vert y\Vert +\Vert x\Vert \Vert k\Vert +\Vert h\Vert \Vert k\Vert )$, which tends to zero as $(h,k)\to (0,0)$. Thus the product bound is equivalent to joint continuity. This is boundedness criterion for a bilinear map.