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Question

If $x^4 + x^2y^2 + y^4 = 8$ and $x^2 + xy + y^2 = 4$, then what is the value of $xy$ ?

The correct answer is
1

To solve the given problem, let's analyze the provided equations:

  • The first equation is \(x^4 + x^2y^2 + y^4 = 8\).
  • The second equation is \(x^2 + xy + y^2 = 4\).

We need to find the value of \(xy\).

Let's solve this step-by-step:

  1. Consider the identity: \((x^2 + y^2)^2 = x^4 + 2x^2y^2 + y^4\).
  2. Let's express \(x^4 + x^2y^2 + y^4\) using this identity:
    • We can rewrite it as: \(x^4 + x^2y^2 + y^4 = (x^2 + y^2)^2 - x^2y^2\).
  3. From the second equation \(x^2 + xy + y^2 = 4\), we know \(x^2 + y^2\) can be expressed in terms of \(xy\):
    • Squaring both sides of \(x^2 + xy + y^2 = 4\), we get:
    • \((x^2 + xy + y^2)^2 = 16\)
    • Expand to find:
      \(x^4 + 2x^3y + 3x^2y^2 + 2xy^3 + y^4 = 16\)
       
  4. Now, notice:
    \(x^4 + x^2y^2 + y^4 = (x^2 + y^2)^2 - x^2y^2 = 8\)
    \((x^2 + y^2)^2 = 8 + x^2y^2\) (from the given equation).
  5. Let \(s = x^2 + y^2\). Then \(s = 4 - xy\). From the identity:
    \((4 - xy)^2 = 8 + x^2y^2\) (substituting).
  6. Let's solve for \(xy\):
    • \(16 - 8xy + (xy)^2 = 8 + (xy)^2\)
    • Cancel \((xy)^2\) from both sides, then \(16 - 8xy = 8\).
    • Simplify to find \(8 = 8xy\).
    • Finally, \(xy = 1\).

Thus, the value of \(xy\) is 1. The correct answer is option: 1.

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Important Questions from Identities

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