$\"\"$

The approximation is $\"\"$ .

Linear approximation of in terms of differential is $\"\"$ .

$\"\"$ .

$\"\"$

Consider $\"\"$ .

Differentiate on each with respect to $\"\"$ .

$\"\"$ .

$\"\"$

Here $\"\"$ , $\"\"$ and $\"\"$ .

$\"\"$

The approximation is reasonable as $\"\"$ is a small value.

$\"\"$ .