28. Find the value of $a$ such that the two equations $x^{2}+a x+1=0$ and $x^{2}-x-a=0$ have one common real root.
28. Ans: 2 .
Using $a x+1=-x-a$, we have $x=-1$ or $a=-1$. But when $a=1$, the original equation has no real roots. Thus $x=-1$, we have $a=2$.