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Inverse Trigonometric Functions question

2019 · 11 Jan · Shift 2 · Q41
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Inverse Trigonometric Functions question

2019 · 11 Jan · Shift 2 · Q41

JEE MainMathematicsInverse Trigonometric FunctionsMCQ+4 / −1
All x satisfying the inequality (cot–1 x)2– 7(cot–1 x) + 10 > 0, lie in the interval :
  1. A
    (cot 2, ∞\infty∞)
  2. B
    (–∞\infty∞, cot 5) ∪\cup∪ (cot 2, ∞\infty∞)
  3. C
    (cot 5, cot 4)
  4. D
    (– ∞\infty∞, cot 5) ∪\cup∪ (cot 4, cot 2)
View written solutionFree

Correct answer: A

  1. Let y=cot⁡−1xy = \cot^{-1}xy=cot−1x

    Then the inequality becomes y2−7y+10>0y^2 - 7y + 10 > 0y2−7y+10>0

  2. Factorize the quadratic y2−7y+10=(y−5)(y−2)y^2 - 7y + 10 = (y-5)(y-2)y2−7y+10=(y−5)(y−2)

    So we need (y−5)(y−2)>0(y-5)(y-2) > 0(y−5)(y−2)>0

  3. Solve the quadratic inequality in yyy

    Since the parabola opens upward, the product is positive outside the roots: y<2ory>5y<2 \quad \text{or} \quad y>5y<2ory>5

  4. Use the principal value range of cot⁡−1x\cot^{-1}xcot−1x

    For JEE, cot⁡−1x∈(0,π)\cot^{-1}x \in (0,\pi)cot−1x∈(0,π)

    Hence 0<y<π0<y<\pi0<y<π

    Now compare with the solution y<2y<2y<2 or y>5y>5y>5.

    Since π≈3.14<5,\pi \approx 3.14 < 5,π≈3.14<5, the condition y>5y>5y>5 is impossible.

    Therefore only 0<y<20<y<20<y<2 remains.

  5. Convert back to xxx

    We have cot⁡−1x<2\cot^{-1}x < 2cot−1x<2

    Since cot⁡y\cot ycoty is strictly decreasing on (0,π)(0,\pi)(0,π), from y<2y<2y<2 we get x=cot⁡y>cot⁡2x = \cot y > \cot 2x=coty>cot2

    Thus the solution set is x∈(cot⁡2,∞)x \in (\cot 2,\infty)x∈(cot2,∞)

  6. Match with options

    This is Option A.


Verification with stored answer

Stored correct answer: A

Our derived answer: A

So the answer agrees with the stored answer.

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