Let $C = (3, 5)$ be the midpoint of $AB,$ where $A = (1, 8)$ and $B = (x, y).$ What is the value of $xy$?
Let's use the midpoint formula. Alright. The midpoint formula is $\left(\frac{x_1+x_2}{2},\frac{y_1+y_2}{2}\right)$. So we can say that $\left(\frac{1+x}{2},\frac{8+y}{2}\right)=(3,5)$. So we get the system of equations $\frac{1+x}{2}=3$ and $\frac{8+y}{2}=5$. This simplifies to $1+x=6$ and $8+y=10$. So we have $x=5$ and $y=2$. And $xy$ is equal to $5*2=10$.

