Domain of Real-Valued Function

XAT 2022 · Quantitative Ability & Data Interpretation · Medium · Functions

Consider the real-valued function f(x)=log⁡(3x−7)2x2−7x+6f(x) = \frac{\log (3x-7)}{\sqrt{2x^2 - 7x + 6}}. Find the domain of f(x)f(x).

  1. A.

    (\frac{7}{3}, \infty)

  2. B.

    \mathbb{R} - {\frac{7}{3}}

  3. C.

    \mathbb{R} - {\frac{3}{2}, 2}

  4. D.

    \mathbb{R} - {\frac{3}{2}, 2, \frac{7}{3}}

  5. E.

    (-\infty, \frac{7}{3})

Answer

A

Explanation

  1. Numerator log⁡(3x−7)\log(3x-7) requires 3x−7>0  ⟹  x>7/3≈2.333x - 7 > 0 \implies x > 7/3 \approx 2.33.
  2. Denominator 2x2−7x+6\sqrt{2x^2 - 7x + 6} requires 2x2−7x+6>02x^2 - 7x + 6 > 0. Roots of 2x2−7x+6=02x^2 - 7x + 6 = 0 are x=3/2x = 3/2 and x=2x = 2. So 2x2−7x+6>02x^2 - 7x + 6 > 0 for x<3/2x < 3/2 or x>2x > 2. Since 7/3>27/3 > 2, for all x>7/3x > 7/3, 2x2−7x+62x^2 - 7x + 6 is strictly positive. Thus, domain is (7/3,∞)(7/3, \infty).

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