The complete solution set of the inequality [cot−1x]2−6[cot−1x]+9≤0, where [.] denotes greatest integer function is
(−∞,cot2)
(−∞,cot3)
(cot2,cot3)
(cot2,∞)
[cot−1x]2−6[cot−1x]+9≤0 ⇒([cot−1x]−3)2≥0∴[cot−1x]=3⇒3≤cot−1x<4x∈(−∞,cot3)