The number of apples produced by each tree in an apple orchard depends on how densely the trees are planted. If $n$ trees are planted on an acre of land, then each tree produces $900-9 n$ apples. So the number of apples produced per acre is
$$
A(n)=n(900-9 n)
$$
How many trees should be planted per acre to obtain the maximum yield of apples?
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