If x + y + 3 = 0, then find the value of x3 + y3 - 9xy + 9.

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SSC CGL 2021 Tier-I (Held On : 11 April 2022 Shift 1)
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  1. -36
  2. -18
  3. 36
  4. 18

Answer (Detailed Solution Below)

Option 2 : -18
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Detailed Solution

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Given:

x + y + 3 = 0

Formula used:

(a + b)3 = a3 + b3 + 3ab (a + b)

Calculation:

x + y + 3 = 0

⇒ x + y = - 3   .....(1)

⇒ (x + y)3 = (- 3)3  [Taking cube of both sides]

⇒ x3 + y3 + 3xy (x + y) = - 27

⇒ x3 + y3 + 3xy × (- 3) = - 27  [∵ x + y = - 3]

⇒ x3 + y- 9xy = - 27

⇒ x3 + y- 9xy + 9 = - 27 + 9  [Adding 9 in both sides]

⇒ x3 + y- 9xy + 9 = - 18

∴ The value of x3 + y- 9xy + 9 is (- 18)

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