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(a).

The revunue is $\"\"$ .

The general form of quadratic equation is $\"\"$ .

Consider the two equations $\"\"$ , $\"\"$ .

Vertex is $\"\"$

Axis of symmetry $\"\"$

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Axis of symmetry is $\"\"$

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The maximum revenue is obtained when the unit price is $\"\"$ dollars.

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The maximum revenue is obtained when the unit price is $\"\"$ dollars.