Let $C = (3, 5)$ be the midpoint of $AB,$ where $A = (1, 8)$ and $B = (x, y).$ What is the value of $xy$?

From the midpoint formula, B = (2,13/2) so xy = 2*13/2 = 13.

Using midpoint formula, \(({{x + 1} \over 2}, {{y + 1} \over 2}) = (3, 5)\), so x = 5 and y = 9, so xy = 45