Question:

\(N\) denotes the set of all natural numbers. Define, for \(a \in N\) \[ aN=\{ax:x\in N\} \] Then \[ 5N \cap 3N= \]

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The intersection of \(aN\) and \(bN\) is always the set of multiples of \(\text{LCM}(a,b)\).
Updated On: Jul 15, 2026
  • \(5N\)
  • \(3N\)
  • \(15N\)
  • \(\emptyset\)
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The Correct Option is C

Solution and Explanation

Concept: Intersection means common elements. \[ 5N=\{5,10,15,20,25,\dots\} \] \[ 3N=\{3,6,9,12,15,18,21,\dots\} \]

Step 1:
Find common multiples.
Common elements are: \[ 15,30,45,60,\dots \] These are exactly the multiples of: \[ LCM(5,3)=15 \] So: \[ 5N\cap 3N=15N \] Thus, the required answer is: \[ \boxed{15N} \]
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