Given x = (√2 + 1)/(√2 − 1) and x − y = 4√2, determine the exact value of x^2 + y^2.

Difficulty: Medium

Correct Answer: 34

Explanation:


Introduction / Context:
This problem combines rationalizing a radical fraction and manipulating sums/differences of two variables. The key is to simplify x first, then exploit the given difference x − y to identify y and compute x^2 + y^2 efficiently.



Given Data / Assumptions:

  • x = (√2 + 1)/(√2 − 1).
  • x − y = 4√2.
  • All square roots are principal (non-negative) values.


Concept / Approach:
Rationalize x by multiplying numerator and denominator by the conjugate (√2 + 1). Then, given the numerical value of x, use x − y = 4√2 to obtain y. Finally, compute x^2 + y^2 by squaring and adding; conjugate-like structures will cancel mixed terms.



Step-by-Step Solution:

x = (√2 + 1)/(√2 − 1) * (√2 + 1)/(√2 + 1) = (√2 + 1)^2 / (2 − 1) = (2 + 2√2 + 1) / 1 = 3 + 2√2.Given x − y = 4√2, solve for y: y = x − 4√2 = (3 + 2√2) − 4√2 = 3 − 2√2.Compute x^2 = (3 + 2√2)^2 = 17 + 12√2; y^2 = (3 − 2√2)^2 = 17 − 12√2.Sum: x^2 + y^2 = (17 + 12√2) + (17 − 12√2) = 34.


Verification / Alternative check:
Observe that y = 3 − 2√2 satisfies x − y = 4√2 directly; the conjugate structure guarantees mixed terms cancel when squaring and adding.



Why Other Options Are Wrong:
38, 32, 30, and 28 are typical miscomputations from squaring or handling radicals improperly; only 34 is consistent.



Common Pitfalls:
Not rationalizing correctly, or attempting to compute y via a guess without ensuring the exact difference equals 4√2.



Final Answer:
34

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