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Previous Year Question (PYQs)



Find the cardinality of the set C which is defined as $C={\{x|\, \sin 4x=\frac{1}{2}\, forx\in(-9\pi,3\pi)}\}$.





Solution

We are given:

\[ \sin(4x) = \frac{1}{2}, \quad x \in (-9\pi,\ 3\pi) \]

Step 1: General solutions for \( \sin(θ) = \frac{1}{2} \)

\[ θ = \frac{\pi}{6} + 2n\pi \quad \text{or} \quad θ = \frac{5\pi}{6} + 2n\pi \]

Let \( θ = 4x \), so we get:

  • \( x = \frac{\pi}{24} + \frac{n\pi}{2} \)
  • \( x = \frac{5\pi}{24} + \frac{n\pi}{2} \)

✅ Step 2: Count how many such \( x \) fall in the interval \( (-9\pi, 3\pi) \)

By checking all possible \( n \) values, we find:

  • For \( x = \frac{\pi}{24} + \frac{n\pi}{2} \): 24 valid values
  • For \( x = \frac{5\pi}{24} + \frac{n\pi}{2} \): 24 valid values

? Total distinct values = 24 + 24 = 48

✅ Final Answer: $\boxed{48}$



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