Question:medium

If the domain of the function
\[ f(x)=\sin^{-1}\!\left(\frac{5-x}{3+2x}\right)+\frac{1}{\log_e(10-x)} \]
is \( (-\infty,\alpha] \cup [\beta,\gamma) - \{\delta\} \), then \( 6(\alpha+\beta+\gamma+\delta) \) is equal to

Show Hint

When finding the domain of composite functions, always evaluate each term separately and then take the intersection of all valid intervals.
Updated On: Mar 5, 2026
  • 68
  • 66
  • 70
  • 67
Show Solution

The Correct Option is C

Solution and Explanation

To find the domain of the given function \(f(x)=\sin^{-1}\!\left(\frac{5-x}{3+2x}\right)+\frac{1}{\log_e(10-x)}\), we need to consider the domain restrictions of both components separately: \(\sin^{-1}(u)\) and \(\frac{1}{\log_e(10-x)}\).

  1. Domain of \(\sin^{-1}\!\left(\frac{5-x}{3+2x}\right)\):
    • The argument of \(\sin^{-1}(x)\) should lie between -1 and 1: \(-1 \leq \frac{5-x}{3+2x} \leq 1\).
    • Solving \(-\frac{5-x}{3+2x} \leq 1\):
      • Simplifying, \(-5+x \geq 3+2x \Rightarrow -5-3 \geq 2x-x\),
      • Thus, \(-8 \geq x \Rightarrow x \leq -8\)
    • Solving \(\frac{5-x}{3+2x} \leq 1\):
      • Expanding, \(5-x \leq 3+2x \Rightarrow 5-3 \leq 2x+x\),
      • Results in \(2 \leq 3x \Rightarrow x \geq \frac{2}{3}\).
    • Therefore, the domain from sine inverse is \(x \leq -8\) and \(x \geq \frac{2}{3}\).
  2. Domain of \(\frac{1}{\log_e(10-x)}\):
    • The logarithm is defined when \(10-x > 0\) giving \(x < 10\).
    • Additionally, we must have \(\log_e(10-x) \neq 0\), i.e., \(10-x \neq 1 \Rightarrow x \neq 9\).
  3. Combining the results:
    • The domain is \((-\infty, -8]\cup[\frac{2}{3}, 10)\)
    • Excluding \(x=9\), the domain becomes \((-\infty, -8]\cup[\frac{2}{3},10)-\{9\}\).
    • Identifying values: \(\alpha = -8\)\(\beta = \frac{2}{3}\)\(\gamma = 10\)\(\delta = 9\).

Finally, calculate: \(6(\alpha + \beta + \gamma + \delta)\)

  • \(6(-8 + \frac{2}{3} + 10 + 9)\)
  • \(6(\frac{-24}{3} + \frac{2}{3} + \frac{30}{3} + \frac{27}{3})\)
  • \(6(\frac{35}{3})\)
  • Evaluating: \(6 \times \frac{35}{3} = 2 \times 35 = 70\).
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