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For x + y = 10 with x, y > 0, the product x²y is greatest when x =
A10/3
B20/3
C5
D8
Answer & Solution
Correct answer: B. 20/3
f = x²(10 − x) has f' = 20x − 3x², zero at x = 20/3, and f'' < 0 there.
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