For what value of $4 \cdot 16 b$ do the equations $1988 x^{2}+b x+8891=0$ and $8891 x^{2}+b x+1988=0$ have a common root.
［Solution］Suppose the given two equations have a common root $x$, then
$$
1988 x^{2}+b x+8891=8891 x^{2}+b x+1988 \text {, }
$$

Simplifying, we get $x= \pm 1$. Substituting $x$ back into the given equations, we obtain
$$
b=\mp 10879 .
$$