In the corner of a field is seen a milkmaid milking a cow, and on the other side of the field is the dairy where the extract has to be deposited. 
Draw a line from the milking-stool down to the river and thence to the door of the dairy, which shall indicate the shortest possible route for the milkmaid. 
It is quite easy to indicate the exact spot on the bank of the river to which she should direct her steps if she wants as short a walk as possible. 

Can you find that spot?

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Math Genius