Answer
$8.9944$
Work Step by Step
The expression for $y$ can be written as: $y=x^{0.5}$
Now, $\dfrac{dy}{dx}=0.5x^{-0.5}$
$dy=0.5x^{-0.5} dx$
Next, we have: $dy=0.5 \times (81)^{-0.5}(-0.1)=-0.005556$
So, the new $y$ value is equal to $-0.005556+(81)^{0.5}=8.9944$