calculus
posted by mark11 .
use implicit differentiation to find dy/dx . Then evaluate the derivative function at the designated point.
log (xy)= 2; (4, 25)

log(xy) = 2
1/(xy) (y + xy') = 0
1/x + 1/y y' = 0
y' = y/x
Or, going about it another way,
logx + logy = 2
1/x + 1/y y' = 0
y' = y/x
So, plug in your numbers.