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<h2>Check the explanation.</h2>Step-by-step explanation:
The standard form of an ellipse is .
The span of the river is 20 meters. As per the given condition, y-axis will pass through the middle point of the river. That is why, the value of a will be .
Since the height of the arc is 6, the value of b is 6.
The equation of the arc will become .