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Tangent through a point

A portion of a river has the shape of the equation y=1-x^2/4, where distances are measured in tens of kilometres, and the positive y-axis represents due north. the town of Coopers Crossing is situated on the river at its most northerly point. The town of Black Stump is 10 kilometres due south of Coopers Crossing. the town of Andrewsville is 25 kilometres east of Black Stump. It is planned to build another town, Macquarie Flats, due north of Coopers Crossing and a road will be built connecting Macquarie Flats to Andrewsville. It has been stipulated that this road must be in a straight line and must not cross the river. What is the shortest length of road (to the nearest kilometre)?

(HINT- Find the co-ordinates of the points representing Coopers Crossing, Black Stump and Andrewsville and draw a diagram. Remember that 1 unit represents 10 kilometres. The solution has something to do with finding a tangent through a certain point.)

Solution Summary

This shows how to find shortest length of road by computing tangent through a point.