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Question
If x3 + y3 = 9 and x + y = 3, find xy.
Sum
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Solution
x3 + y3 = 9, x + y = 3
(x + y)3 = x3 + y3 + 3xy (x + y)
⇒ (3)3 = 9 + 3xy (3)
⇒ 27 = 9 + 9xy
⇒ 9xy = 27 - 9 = 18
⇒ xy = 2.
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