$\"\"$

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

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Observe the figure the plank forms right angle triangle.

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

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

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From pythagoras rule $\"\"$ .

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

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$\"\"$

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$\"\"$

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

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$\"\"$

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$\"\"$

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

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$\"\"$

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$\"\"$

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(a).

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

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$\"\"$

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$\"\"$ .

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

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$\"\"$

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$\"\"$

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$\"\"$

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$\"\"$ .