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NCERT Exemplar · Q25

Q.If [x]2−5[x]+6=0[x]^2 - 5[x] + 6 = 0, where [ . ][\,.\,] denote the greatest integer function, then
(A) x∈[3,4]x \in [3, 4]
(B) x∈(2,3]x \in (2, 3]
(C) x∈[2,3]x \in [2, 3]
(D) x∈[2,4)x \in [2, 4)

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[x][x] is an integer; solving [x]2−5[x]+6=0[x]^2-5[x]+6=0 gives [x]=2[x]=2 or [x]=3[x]=3, so x∈[2,3)∪[3,4)=[2,4)x\in[2,3)\cup[3,4)=[2,4) — option (D).

The greatest integer function [x][x] returns the largest integer not exceeding xx, so [x][x] is always an integer. Let [x]=n[x]=n with n∈Zn\in\mathbb{Z}.

Step 1 — Solve the quadratic in nn.

n2−5n+6=0 ⇒ (n−2)(n−3)=0 ⇒ n=2 or n=3n^2-5n+6=0\ \Rightarrow\ (n-2)(n-3)=0\ \Rightarrow\ n=2\ \text{or}\ n=3 …

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