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  1. If xy (x + y) = 1, then the value of
    1
    		 - x3 - y3 is :
    x3y3
    1. 0
    2. 1
    3. 3
    4. –2
Correct Option: C

xy (x + y) = 1

⇒ x + y =
1
xy

On cubing both sides,
⇒ x3 + y3 + 3xy (x + y) =
1
x3y3

⇒ x3 + y3 + 3xy ×
1
 =
1
xyx3y3

⇒
1
 - x3 - y3 = 3
x3y3


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