Question : If $x - y = 8$ and $8xy = 8$, then what is the value of $2 x^3-2 y^3$?
Option 1: 536
Option 2: 1092
Option 3: 1072
Option 4: 1608
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Correct Answer: 1072
Solution :
$8xy = 8$
$⇒ xy = 1$
and $x - y = 8$
Cubing both sides, we get,
$(x - y)^3 = 8^3$
⇒ $x^3 - y^3 - 3xy(x - y) = 512$
Putting the given values, we get,
⇒ $x^3 - y^3 - 3 \times 1 \times 8 = 512$
⇒ $x^3 - y^3 = 24 + 512$
⇒ $x^3 - y^3 = 536$
$\therefore2x^3 - 2y^3 = 536 \times 2 = 1072$
Hence, the correct answer is 1072.
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