Hello everybody. I'm getting headache of not getting the right result.

A trough of length has a cross section in the shape of a semicircle with radius . When filled with water to within a distance of the top, the volume of water is

Suppose ft, ft, and . Find the depth of water in the trough to within 0.01 ft.

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Now I'm supposed to find from this equation

and my solution is approx. 0.16(and I know it's right for the above equation) while the answer in book is 'the depth of water is 0.838 ft.'