2. In each cell of a $2 \times 2$ table, one number is written. All numbers are distinct, the sum of the numbers in the first row is equal to the sum of the numbers in the second row, and the product of the numbers in the first column is equal to the product of the numbers in the second column. Find the sum of all four numbers.
Answer: 0. Solution: Let the numbers in the top row of the table be (from left to right) $a$ and $b$, and the numbers in the bottom row (from left to right) be $c$ and $d$. By the condition, $a+b=c+d$ and $a c=b d$. Expressing $c$ from the first equation and substituting into the second, we get $a(a+b-d)=b d \Leftrightarrow(a+b)(a-d)=0$, from which, since $a-d \neq 0$, we obtain $a=-b$. Similarly, $d=-c$, from which we get the answer.