$\"\"$

\

The length of plank is $\"\"$ .

\

Observe the figure the plank forms right angle triangle.

\

Consider length of wall is $\"\"$ and base of the triangle is $\"\"$

\

And rate of change in $\"\"$ is $\"\"$ .

\

From pythagoras rule $\"\"$

\

Apply derivative on each side with respect to $\"\"$ .

\

$\"\"$

\

$\"\"$

\

From the power rule of derivative: $\"\"$ .

\

$\"\"$

\

$\"\"$

\

Substitute $\"\"$ in the above equation.

\

$\"\"$

\

$\"\"$

\

(a).

\

Find $\"\"$ when $\"\"$ .

\

$\"\"$

\

$\"\"$ .

\

Substitute $\"\"$ and $\"\"$ in the $\"\"$ .

\

$\"\"$

\

$\"\"$

\

$\"\"$

\

$\"\"$ .