4. The numbers $x$ and $y$ are such that $x+y=xy=17$. Find the value of the expression:

$$
\left(x^{2}-17 x\right)\left(y+\frac{17}{y}\right)
$$
Answer: -289

Solution. $x-17=-y, \frac{17}{y}=x$. The desired expression is:

$$
x(x-17)(y+x)=-x y \cdot 17=-289 .
$$

Criteria.

$(-)$

$(-$.

(-/+) Serious arithmetic error affecting the solution process, or several arithmetic errors.

$(+/ 2)$

(+/-) One arithmetic error in an otherwise correct solution (for example, incorrectly multiplying 17 and 17 in the last step).

(+.) Omission of minor calculations.

(+)