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Inverse Trigonometric Functions question

2021 · 1 Sep · Shift 2 · Q25
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  5. /2021 · 1 Sep · Shift 2 · Q25

Inverse Trigonometric Functions question

2021 · 1 Sep · Shift 2 · Q25

JEE MainMathematicsInverse Trigonometric FunctionsMCQ+4 / −1
cos⁡−1(cos⁡(−5))+sin⁡−1(sin⁡(6))−tan⁡−1(tan⁡(12)){\cos ^{ - 1}}(\cos ( - 5)) + {\sin ^{ - 1}}(\sin (6)) - {\tan ^{ - 1}}(\tan (12))cos−1(cos(−5))+sin−1(sin(6))−tan−1(tan(12)) is equal to : (The inverse trigonometric functions take the principal values)
  1. A
    3 π−\pi-π− 11
  2. B
    4 π−\pi-π− 9
  3. C
    4 π−\pi-π− 11
  4. D
    3 π\piπ + 1
View written solutionFree

Correct answer: NONE OF THE OPTIONS IS CORRECT; THE EXPRESSION SIMPLIFIES TO \(8\PI-23\).

  1. Use principal value ranges

For inverse trigonometric functions:

  • cos⁡−1x∈[0,π]\cos^{-1}x \in [0,\pi]cos−1x∈[0,π]
  • sin⁡−1x∈[−π2,π2]\sin^{-1}x \in \left[-\frac{\pi}{2},\frac{\pi}{2}\right]sin−1x∈[−2π​,2π​]
  • tan⁡−1x∈(−π2,π2)\tan^{-1}x \in \left(-\frac{\pi}{2},\frac{\pi}{2}\right)tan−1x∈(−2π​,2π​)

We need to evaluate

cos⁡−1(cos⁡(−5))+sin⁡−1(sin⁡6)−tan⁡−1(tan⁡12).\cos^{-1}(\cos(-5)) + \sin^{-1}(\sin 6) - \tan^{-1}(\tan 12).cos−1(cos(−5))+sin−1(sin6)−tan−1(tan12).
  1. Evaluate cos⁡−1(cos⁡(−5))\cos^{-1}(\cos(-5))cos−1(cos(−5))

Since cosine is even,

cos⁡(−5)=cos⁡5.\cos(-5)=\cos 5.cos(−5)=cos5.

So,

cos⁡−1(cos⁡(−5))=cos⁡−1(cos⁡5).\cos^{-1}(\cos(-5))=\cos^{-1}(\cos 5).cos−1(cos(−5))=cos−1(cos5).

Now 5∈[π,2π]5 \in [\pi,2\pi]5∈[π,2π], and for x∈[π,2π]x \in [\pi,2\pi]x∈[π,2π],

cos⁡−1(cos⁡x)=2π−x.\cos^{-1}(\cos x)=2\pi-x.cos−1(cosx)=2π−x.

Hence,

cos⁡−1(cos⁡5)=2π−5.\cos^{-1}(\cos 5)=2\pi-5.cos−1(cos5)=2π−5.
  1. Evaluate sin⁡−1(sin⁡6)\sin^{-1}(\sin 6)sin−1(sin6)

We want the principal value in [−π2,π2]\left[-\frac{\pi}{2},\frac{\pi}{2}\right][−2π​,2π​].

Now,

6∈[3π2,2π]6 \in \left[\frac{3\pi}{2},2\pi\right]6∈[23π​,2π]

because

3π2≈4.712,2π≈6.283.\frac{3\pi}{2}\approx 4.712, \qquad 2\pi\approx 6.283.23π​≈4.712,2π≈6.283.

For x∈[3π2,2π]x \in \left[\frac{3\pi}{2},2\pi\right]x∈[23π​,2π],

sin⁡−1(sin⁡x)=2π−x.\sin^{-1}(\sin x)=2\pi-x.sin−1(sinx)=2π−x.

Therefore,

sin⁡−1(sin⁡6)=2π−6.\sin^{-1}(\sin 6)=2\pi-6.sin−1(sin6)=2π−6.
  1. Evaluate tan⁡−1(tan⁡12)\tan^{-1}(\tan 12)tan−1(tan12)

The principal value of tan⁡−1\tan^{-1}tan−1 lies in

(−π2,π2).\left(-\frac{\pi}{2},\frac{\pi}{2}\right).(−2π​,2π​).

Since tangent has period π\piπ, reduce 121212 by multiples of π\piπ.

We note that

12−4π∈(−π2,π2)12-4\pi \in \left(-\frac\pi2,\frac\pi2\right)12−4π∈(−2π​,2π​)

since

4π≈12.566,4\pi \approx 12.566,4π≈12.566,

so

12−4π≈−0.566,12-4\pi \approx -0.566,12−4π≈−0.566,

which is indeed in the principal interval. Hence,

tan⁡−1(tan⁡12)=12−4π.\tan^{-1}(\tan 12)=12-4\pi.tan−1(tan12)=12−4π.
  1. Substitute all values

So the expression becomes

(2π−5)+(2π−6)−(12−4π).(2\pi-5)+(2\pi-6)-(12-4\pi).(2π−5)+(2π−6)−(12−4π).

Simplify:

2π−5+2π−6−12+4π2\pi-5+2\pi-6-12+4\pi2π−5+2π−6−12+4π =8π−23.=8\pi-23.=8π−23.
  1. Match with the options

Given options are:

  • A: 3π−113\pi-113π−11
  • B: 4π−94\pi-94π−9
  • C: 4π−114\pi-114π−11
  • D: 3π+13\pi+13π+1

Our result is

8π−23.8\pi-23.8π−23.

Now check whether it equals any option modulo nothing (since this is a direct value, not periodic after inverse functions):

8π−23≈2.133,8\pi-23 \approx 2.133,8π−23≈2.133,

while

4π−11≈1.566.4\pi-11 \approx 1.566.4π−11≈1.566.

So it does not match option C, and in fact matches none of the given options.


  1. Conclusion

The correct value is

8π−23.\boxed{8\pi-23}.8π−23​.

Therefore, the stored answer appears to be incorrect.

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