Question

If xx is real and ∣x2−2x+3∣=11|x^2 - 2x + 3| = 11, then possible values of ∣−x3+x2−x∣|-x^3 + x^2 - x| include

Choose one option
Official answer and solution
Official answer
D
From
Official GATE 2014 answer key

Solution

Solve the absolute value quadratic equation for real x and evaluate the target expression for each valid root.

Given ∣x2−2x+3∣=11|x^2 - 2x + 3| = 11.

  1. Analyze the Quadratic:

    x2−2x+3=(x−1)2+2.x^2 - 2x + 3 = (x - 1)^2 + 2.

    For all real xx, (x−1)2≥0(x - 1)^2 \ge 0, which implies x2−2x+3≥2>0x^2 - 2x + 3 \ge 2 > 0.
    Because the term inside the absolute value is strictly positive, the equation simplifies to:

    x2−2x+3=11  ⟹  x2−2x−8=0.x^2 - 2x + 3 = 11 \implies x^2 - 2x - 8 = 0.
  2. Solve for xx:

    (x−4)(x+2)=0  ⟹  x=4orx=−2.(x - 4)(x + 2) = 0 \implies x = 4 \quad \text{or} \quad x = -2.
  3. Evaluate Target Expression ∣−x3+x2−x∣|-x^3 + x^2 - x|:

    • Case 1 (x=4x = 4):
      −43+42−4=−64+16−4=−52.-4^3 + 4^2 - 4 = -64 + 16 - 4 = -52.
      ∣−52∣=52.|-52| = 52.
    • Case 2 (x=−2x = -2):
      −(−2)3+(−2)2−(−2)=−(−8)+4+2=8+4+2=14.-(-2)^3 + (-2)^2 - (-2) = -(-8) + 4 + 2 = 8 + 4 + 2 = 14.
      ∣14∣=14.|14| = 14.
  4. Thus, the possible values are 1414 and 5252 (option D).

Final answer

D

Common trap

Missing the case x = -2 or making sign errors when substituting negative values.

The answer is the one in the official GATE answer key. The solution is this site's own explanation.

Source

Paper
GATE 2014 Computer Science and Information Technology (CS) Session 2
Question
8 of 65, printed as GA Q.8
Section
General Aptitude

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