5. Given $\cos ^{2}(\alpha+\beta)-\cos ^{2}(\alpha-\beta)=x, \cos ^{2} \alpha$
$\cdot \cos ^{2} \beta=y, y \neq 0$. Then express $\operatorname{tg} \alpha \operatorname{ctg} \beta=$ $\qquad$
5. $-\frac{x}{4 y}$