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NCERT Exemplar · Q80

Q.lim⁡x→3+x[x]=\lim_{x \to 3^+} \dfrac{x}{[x]} = ________.

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The limit of a rational function where the denominator is the greatest integer function [x][x] as x→3+x \to 3^+ is found by evaluating the function just to the right of 3. Since [x]=3[x] = 3 for x∈(3,4)x \in (3, 4), the limit simplifies to 33=1\frac{3}{3} = 1.

The key here is understanding what [x][x] means — the greatest integer less than or equal to xx, also called the floor function. For any xx just greater than 3, say x=3.1x = 3.1, [x]=3[x] = 3. This is constant on the interval (3,4)(3, 4), so the denominator behaves like a fixed number near x=3+x = 3^+.

Because the numerator xx approaches 3, and the denominator is exactly 3 for all xx in a right-neighbourhood of 3, the limit is straightforward — no factoring, no cancellation, just direct substitution into a constant denominator.

  1. Identify the behaviour of [x][x] near x=3+x = 3^+.

    For xx in (3,4)(3, 4), the greatest integer less than or equal to xx is 3. So [x]=3[x] = 3 for all xx with 3<x<43 < x < 4.

  2. Rewrite the function for xx just to the right of 3.

    Since [x]=3[x] = 3 in that region, we have

x[x]=x3for 3<x<4.\frac{x}{[x]} = \frac{x}{3} \quad \text{for } 3 < x < 4.

  1. Take the limit as x→3+x \to 3^+. Now the limit becomes

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