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Functions question

2018 · Shift 2 · Q33
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Functions question

2018 · Shift 2 · Q33

JEE AdvancedMathematicsFunctionsMCQ+3 / −1
Let E1={x∈R:x≠1 and xx−1>0}E_1 = \left\{ x \in \mathbb{R} : x \ne 1 \text{ and } \frac{x}{x-1} \gt 0 \right\}E1​={x∈R:x=1 and x−1x​>0} and E2={x∈E1:sin⁡−1(log⁡e(xx−1)) is a real number}E_2 = \left\{ x \in E_1 : \sin^{-1}\left(\log_e\left(\frac{x}{x-1}\right)\right) \text{ is a real number} \right\}E2​={x∈E1​:sin−1(loge​(x−1x​)) is a real number}(Here, the inverse trigonometric function sin⁡−1x\sin^{-1} xsin−1x assumes values in [−π2,π2]\left[-\frac{\pi}{2}, \frac{\pi}{2}\right][−2π​,2π​].) Let f:E1→Rf : E_1 \to \mathbb{R}f:E1​→R be the function defined by f(x)=log⁡e(xx−1)f(x) = \log_e\left(\frac{x}{x-1}\right)f(x)=loge​(x−1x​) and g:E2→Rg : E_2 \to \mathbb{R}g:E2​→R be the function defined by g(x)=sin⁡−1(log⁡e(xx−1))g(x) = \sin^{-1}\left(\log_e\left(\frac{x}{x-1}\right)\right)g(x)=sin−1(loge​(x−1x​)). | LIST-I | LIST-II | |--------|--------| | P. The range of fff is | 1. (−∞,11−e]∪[ee−1,∞)\left(-\infty, \frac{1}{1-e}\right] \cup \left[\frac{e}{e-1}, \infty\right)(−∞,1−e1​]∪[e−1e​,∞) | | Q. The range of ggg contains | 2. (0,1)(0, 1)(0,1) | | R. The domain of fff contains | 3. [−12,12]\left[-\frac{1}{2}, \frac{1}{2}\right][−21​,21​] | | S. The domain of ggg is | 4. (−∞,0)∪(0,∞)(-\infty, 0) \cup (0, \infty)(−∞,0)∪(0,∞) | | | 5. (−∞,ee−1)\left(-\infty, \frac{e}{e-1}\right)(−∞,e−1e​) | | | 6. (−∞,0)∪(12,ee−1](-\infty, 0) \cup \left(\frac{1}{2}, \frac{e}{e-1}\right](−∞,0)∪(21​,e−1e​] | The correct option is :
  1. A
    P →\to→ 4; Q →\to→ 2; R →\to→ 1 ; S →\to→ 1
  2. B
    P →\to→ 3; Q →\to→ 3; R →\to→ 6 ; S →\to→ 5
  3. C
    P →\to→ 4; Q →\to→ 2; R →\to→ 1 ; S →\to→ 6
  4. D
    P →\to→ 4; Q →\to→ 3; R →\to→ 6 ; S →\to→ 5
View written solutionFree

Correct answer: NO OPTION IS EXACTLY CORRECT., SUGGESTED CORRECT MATCHING: P -> 4, Q -> 2, R -> 4, S -> 1., HENCE THE STORED ANSWER A SEEMS INCONSISTENT WITH THE MATHEMATICS OF THE PROBLEM.

  1. Find E1E_1E1​ (domain of fff)

We need x≠1andxx−1>0.x\neq 1 \quad \text{and} \quad \frac{x}{x-1}>0.x=1andx−1x​>0.

Now, the fraction is positive when numerator and denominator have the same sign.

  • If x>0x>0x>0 and x−1>0x-1>0x−1>0, then x>1x>1x>1.
  • If x<0x<0x<0 and x−1<0x-1<0x−1<0, then x<0x<0x<0.

Hence, E1=(−∞,0)∪(1,∞).E_1=(-\infty,0)\cup(1,\infty).E1​=(−∞,0)∪(1,∞).

So the domain of fff is (−∞,0)∪(1,∞),(-\infty,0)\cup(1,\infty),(−∞,0)∪(1,∞), which is clearly contained in (−∞,0)∪(0,∞).(-\infty,0)\cup(0,\infty).(−∞,0)∪(0,∞). Thus, R→4.R \to 4.R→4.


  1. Find the range of f(x)=ln⁡(xx−1)f(x)=\ln\left(\frac{x}{x-1}\right)f(x)=ln(x−1x​)

Let y=xx−1=1+1x−1.y=\frac{x}{x-1}=1+\frac{1}{x-1}.y=x−1x​=1+x−11​. We analyze yyy on the domain (−∞,0)∪(1,∞)(-\infty,0)\cup(1,\infty)(−∞,0)∪(1,∞).

  • For x<0x<0x<0: xx−1∈(0,1).\frac{x}{x-1}\in(0,1).x−1x​∈(0,1). Indeed, as x→−∞x\to -\inftyx→−∞, xx−1→1−\frac{x}{x-1}\to 1^{-}x−1x​→1−, and as x→0−x\to 0^{-}x→0−, xx−1→0+\frac{x}{x-1}\to 0^{+}x−1x​→0+.

  • For x>1x>1x>1: xx−1>1.\frac{x}{x-1}>1.x−1x​>1. As x→1+x\to 1^{+}x→1+, xx−1→+∞\frac{x}{x-1}\to +\inftyx−1x​→+∞, and as x→+∞x\to +\inftyx→+∞, it tends to 1+1^{+}1+.

Therefore, xx−1∈(0,1)∪(1,∞).\frac{x}{x-1}\in (0,1)\cup(1,\infty).x−1x​∈(0,1)∪(1,∞). Applying ln⁡\lnln, f(x)=ln⁡(xx−1)∈(−∞,0)∪(0,∞).f(x)=\ln\left(\frac{x}{x-1}\right)\in (-\infty,0)\cup(0,\infty).f(x)=ln(x−1x​)∈(−∞,0)∪(0,∞). So the range of fff is (−∞,0)∪(0,∞),(-\infty,0)\cup(0,\infty),(−∞,0)∪(0,∞), which matches item 4. Thus, P→4.P \to 4.P→4.


  1. Find E2E_2E2​ (domain of ggg)

For g(x)=sin⁡−1(ln⁡(xx−1))g(x)=\sin^{-1}\left(\ln\left(\frac{x}{x-1}\right)\right)g(x)=sin−1(ln(x−1x​)) to be real, we need −1≤ln⁡(xx−1)≤1.-1\le \ln\left(\frac{x}{x-1}\right)\le 1.−1≤ln(x−1x​)≤1. Exponentiating, e−1≤xx−1≤e.e^{-1}\le \frac{x}{x-1}\le e.e−1≤x−1x​≤e.

Now solve this on the domain E1=(−∞,0)∪(1,∞)E_1=(-\infty,0)\cup(1,\infty)E1​=(−∞,0)∪(1,∞).

Case 1: x<0x<0x<0

Here x−1<0x-1<0x−1<0.

(i) Solve xx−1≥e−1\dfrac{x}{x-1}\ge e^{-1}x−1x​≥e−1

Since x−1<0x-1<0x−1<0, inequality reverses on multiplying: x≤e−1(x−1).x\le e^{-1}(x-1).x≤e−1(x−1). So, ex≤x−1ex\le x-1ex≤x−1 x(e−1)≤−1x(e-1)\le -1x(e−1)≤−1 x≤−1e−1=11−e.x\le -\frac{1}{e-1}=\frac{1}{1-e}.x≤−e−11​=1−e1​.

(ii) Solve xx−1≤e\dfrac{x}{x-1}\le ex−1x​≤e

Again x−1<0x-1<0x−1<0, so inequality reverses: x≥e(x−1)x\ge e(x-1)x≥e(x−1) x≥ex−ex\ge ex-ex≥ex−e x(1−e)≥−ex(1-e)\ge -ex(1−e)≥−e x≤ee−1.x\le \frac{e}{e-1}.x≤e−1e​. This is automatically true for all x<0x<0x<0.

Thus for x<0x<0x<0, x≤11−e.x\le \frac{1}{1-e}.x≤1−e1​. So we get (−∞,11−e].(-\infty,\tfrac{1}{1-e}].(−∞,1−e1​].

Case 2: x>1x>1x>1

Here x−1>0x-1>0x−1>0.

(i) Solve xx−1≥e−1\dfrac{x}{x-1}\ge e^{-1}x−1x​≥e−1

Multiply directly: x≥e−1(x−1).x\ge e^{-1}(x-1).x≥e−1(x−1). This is automatically true for all x>1x>1x>1.

(ii) Solve xx−1≤e\dfrac{x}{x-1}\le ex−1x​≤e

Multiply directly: x≤e(x−1)x\le e(x-1)x≤e(x−1) x≤ex−ex\le ex-ex≤ex−e e≤x(e−1)e\le x(e-1)e≤x(e−1) x≥ee−1.x\ge \frac{e}{e-1}.x≥e−1e​.

Thus for x>1x>1x>1, we get [ee−1,∞).\left[\frac{e}{e-1},\infty\right).[e−1e​,∞).

Hence, E2=(−∞,11−e]∪[ee−1,∞).E_2=(-\infty,\tfrac{1}{1-e}]\cup\left[\tfrac{e}{e-1},\infty\right).E2​=(−∞,1−e1​]∪[e−1e​,∞). This matches item 1. Thus, S→1.S \to 1.S→1.


  1. Find the range of ggg

Since g(x)=sin⁡−1(f(x)),g(x)=\sin^{-1}(f(x)),g(x)=sin−1(f(x)), and for x∈E2x\in E_2x∈E2​, we have f(x)∈[−1,1]∖{0}?f(x)\in[-1,1]\setminus\{0\}?f(x)∈[−1,1]∖{0}? Let us check whether 000 occurs.

If f(x)=0,f(x)=0,f(x)=0, then ln⁡(xx−1)=0  ⟹  xx−1=1,\ln\left(\frac{x}{x-1}\right)=0 \implies \frac{x}{x-1}=1,ln(x−1x​)=0⟹x−1x​=1, which gives x=x−1,x=x-1,x=x−1, impossible.

So actually f(E2)=[−1,0)∪(0,1].f(E_2)=[-1,0)\cup(0,1].f(E2​)=[−1,0)∪(0,1]. Applying sin⁡−1\sin^{-1}sin−1, g(E2)=[−π2,0)∪(0,π2].g(E_2)=\left[-\frac{\pi}{2},0\right)\cup\left(0,\frac{\pi}{2}\right].g(E2​)=[−2π​,0)∪(0,2π​].

Now among the listed sets, this range certainly contains (0,1)(0,1)(0,1), because (0,1)⊂(0,π2].(0,1)\subset \left(0,\frac{\pi}{2}\right].(0,1)⊂(0,2π​]. But it does not contain [−12,12]\left[-\frac12,\frac12\right][−21​,21​], since 000 is not in the range.

Hence, Q→2.Q \to 2.Q→2.


  1. Final matching

We have found:

  • P→4P \to 4P→4
  • Q→2Q \to 2Q→2
  • R→4R \to 4R→4 (domain of fff contains item 4)
  • S→1S \to 1S→1

Now compare with options:

  • Option A: P→4,  Q→2,  R→1,  S→1P\to 4,\; Q\to 2,\; R\to 1,\; S\to 1P→4,Q→2,R→1,S→1
  • Option B: wrong
  • Option C: wrong
  • Option D: wrong

There is an inconsistency in the options regarding RRR.

The correct statement for RRR should be R→4R\to 4R→4, because Dom(f)=(−∞,0)∪(1,∞)⊂(−∞,0)∪(0,∞),\text{Dom}(f)=(-\infty,0)\cup(1,\infty)\subset (-\infty,0)\cup(0,\infty),Dom(f)=(−∞,0)∪(1,∞)⊂(−∞,0)∪(0,∞), and item 1 is not contained in the domain of fff since it includes numbers in (0,1](0,1](0,1].

So the mathematically correct matching is: P→4,Q→2,R→4,S→1.P\to 4,\quad Q\to 2,\quad R\to 4,\quad S\to 1.P→4,Q→2,R→4,S→1. Since no option matches this, the stored answer A appears to be incorrect, likely due to a misprint in LIST-II or the options.

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