$\"\"$

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$\"\"$ and $\"\"$ are velocities.

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The angles are $\"\"$ and $\"\"$ .

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Draw a diagram.

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$\"\"$

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Observe the diagram :

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Here $\"\"$ and $\"\"$

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$\"\"$

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$\"\"$

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Time $\"\"$ .

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Here the time is considered for both the man can row in water and walking time.

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$\"\"$

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Differetiate with respect to $\"\"$ .

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$\"\"$

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Equate $\"\"$ to zero.

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$\"\"$

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

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$\"\"$ .

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Thus, least time taken by light waves travelling from $\"\"$ to $\"\"$ is $\"\"$ .

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$\"\"$

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$\"\"$

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Differetiate with respect to $\"\"$ .

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$\"\"$

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$\"\"$

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Thus, the time is minimum time.

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$\"\"$

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Least time taken by light waves travelling from $\"\"$ to $\"\"$ is $\"\"$ .