Question : If $2 x+3 y=9$ and $x y=3$, what is $8 x^3+27 y^3?$
Option 1: $235$
Option 2: $244$
Option 3: $243$
Option 4: $234$
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Correct Answer: $243$
Solution :
Given: $2x+3y=9$ and $xy=3$.
$8 x^3+27 y^3$
$= (2x)^3+(3y)^3$
$= (2x+3y)^3-3×2x×3y (2x+3y)$
$= (9)^3-18xy(9)$
$= 729-18×3×9$
$= 729-486$
$= 243$
Hence, the correct answer is $243$.
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