Sign in
12thPass logo
New chatPYQ LibraryDoubtsRank report
Sign in to see Recents

Your guest activity stays on this device

Sign in to save progress →
Sign in

Inverse Trigonometric Functions question

2013 · Shift 1 · Q31
Guest · filters and generic practice availableBrowsing as a guest · PYQ filters and generic practice are available. Sign in only for personalised features and saved progress.
  1. PYQ Library
  2. /JEE Advanced
  3. /Mathematics
  4. /Inverse Trigonometric Functions
  5. /2013 · Shift 1 · Q31

Inverse Trigonometric Functions question

2013 · Shift 1 · Q31

JEE AdvancedMathematicsInverse Trigonometric FunctionsMCQ+2 / −0.5
The value of cot⁡(∑n=123cot⁡−1(1+∑k=1n2k))\cot \left( {\sum\limits_{n = 1}^{23} {{{\cot }^{ - 1}}} \left( {1 + \sum\limits_{k = 1}^n {2k} } \right)} \right)cot(n=1∑23​cot−1(1+k=1∑n​2k)) is
  1. A
    2325{{23} \over {25}}2523​
  2. B
    2523{{25} \over {23}}2325​
  3. C
    2324{{23} \over {24}}2423​
  4. D
    2423{{24} \over {23}}2324​
View written solutionFree

Correct answer: B

The user wants me to solve the following trigonometric expression: cot⁡(∑n=123cot⁡−1(1+∑k=1n2k))\cot \left( {\sum\limits_{n = 1}^{23} {{{\cot }^{ - 1}}} \left( {1 + \sum\limits_{k = 1}^n {2k} } \right)} \right)cot(n=1∑23​cot−1(1+k=1∑n​2k))

Step 1: Simplify the inner sum. First, we evaluate the summation inside the argument of the inverse cotangent function: ∑k=1n2k=2∑k=1nk=2n(n+1)2=n(n+1)\sum_{k = 1}^n {2k} = 2 \sum_{k=1}^n k = 2 \frac{n(n+1)}{2} = n(n+1)∑k=1n​2k=2∑k=1n​k=22n(n+1)​=n(n+1)

Step 2: Rewrite the expression. Substitute the simplified sum back into the main expression. The expression becomes: cot⁡(∑n=123cot⁡−1(1+n(n+1)))\cot \left( {\sum\limits_{n = 1}^{23} {{{\cot }^{ - 1}}} \left( {1 + n(n+1)} \right)} \right)cot(n=1∑23​cot−1(1+n(n+1)))

Step 3: Convert to inverse tangent. We use the identity cot⁡−1(x)=tan⁡−1(1/x)\cot^{-1}(x) = \tan^{-1}(1/x)cot−1(x)=tan−1(1/x) for x>0x > 0x>0. Since n≥1n \ge 1n≥1, the term 1+n(n+1)1 + n(n+1)1+n(n+1) is always positive. The general term of the summation can be written as: {\cot }^{ - 1}} \left( {1 + n(n+1)} \right) = {\tan }^{ - 1}} \left( \frac{1}{1 + n(n+1)} \right)

Step 4: Use the telescopic series identity for inverse tangent. We use the identity tan⁡−1(y)−tan⁡−1(x)=tan⁡−1(y−x1+xy)\tan^{-1}(y) - \tan^{-1}(x) = \tan^{-1}\left(\frac{y-x}{1+xy}\right)tan−1(y)−tan−1(x)=tan−1(1+xyy−x​). We can rewrite the argument of the inverse tangent as: 11+n(n+1)=(n+1)−n1+n(n+1)\frac{1}{1 + n(n+1)} = \frac{(n+1) - n}{1 + n(n+1)}1+n(n+1)1​=1+n(n+1)(n+1)−n​ This fits the identity with y=n+1y = n+1y=n+1 and x=nx = nx=n. So, we have: {\tan }^{ - 1}} \left( \frac{1}{1 + n(n+1)} \right) = \tan^{-1}(n+1) - \tan^{-1}(n)

Step 5: Evaluate the summation. The summation becomes a telescoping series: S=∑n=123(tan⁡−1(n+1)−tan⁡−1(n))S = \sum_{n=1}^{23} (\tan^{-1}(n+1) - \tan^{-1}(n))S=∑n=123​(tan−1(n+1)−tan−1(n)) Let's write out the first few and the last term of the series: For n=1n=1n=1: tan⁡−1(2)−tan⁡−1(1)\tan^{-1}(2) - \tan^{-1}(1)tan−1(2)−tan−1(1) For n=2n=2n=2: tan⁡−1(3)−tan⁡−1(2)\tan^{-1}(3) - \tan^{-1}(2)tan−1(3)−tan−1(2) For n=3n=3n=3: tan⁡−1(4)−tan⁡−1(3)\tan^{-1}(4) - \tan^{-1}(3)tan−1(4)−tan−1(3) ... For n=23n=23n=23: tan⁡−1(24)−tan⁡−1(23)\tan^{-1}(24) - \tan^{-1}(23)tan−1(24)−tan−1(23)

Summing these terms, all intermediate terms cancel out: S=(tan⁡−1(2)−tan⁡−1(1))+(tan⁡−1(3)−tan⁡−1(2))+⋯+(tan⁡−1(24)−tan⁡−1(23))S = (\tan^{-1}(2) - \tan^{-1}(1)) + (\tan^{-1}(3) - \tan^{-1}(2)) + \dots + (\tan^{-1}(24) - \tan^{-1}(23))S=(tan−1(2)−tan−1(1))+(tan−1(3)−tan−1(2))+⋯+(tan−1(24)−tan−1(23)) S=tan⁡−1(24)−tan⁡−1(1)S = \tan^{-1}(24) - \tan^{-1}(1)S=tan−1(24)−tan−1(1)

Step 6: Simplify the sum. Using the identity tan⁡−1(a)−tan⁡−1(b)=tan⁡−1(a−b1+ab)\tan^{-1}(a) - \tan^{-1}(b) = \tan^{-1}\left(\frac{a-b}{1+ab}\right)tan−1(a)−tan−1(b)=tan−1(1+aba−b​) again: S=tan⁡−1(24−11+24⋅1)=tan⁡−1(2325)S = \tan^{-1}\left(\frac{24 - 1}{1 + 24 \cdot 1}\right) = \tan^{-1}\left(\frac{23}{25}\right)S=tan−1(1+24⋅124−1​)=tan−1(2523​)

Step 7: Calculate the final value. The original expression is cot⁡(S)\cot(S)cot(S). cot⁡(S)=cot⁡(tan⁡−1(2325))\cot(S) = \cot\left(\tan^{-1}\left(\frac{23}{25}\right)\right)cot(S)=cot(tan−1(2523​)) Let θ=tan⁡−1(2325)\theta = \tan^{-1}\left(\frac{23}{25}\right)θ=tan−1(2523​). This implies tan⁡(θ)=2325\tan(\theta) = \frac{23}{25}tan(θ)=2523​. Since cot⁡(θ)=1tan⁡(θ)\cot(\theta) = \frac{1}{\tan(\theta)}cot(θ)=tan(θ)1​, we have: cot⁡(S)=123/25=2523\cot(S) = \frac{1}{23/25} = \frac{25}{23}cot(S)=23/251​=2325​

Thus, the value of the given expression is 2523\frac{25}{23}2325​. This corresponds to option B.

PreviousNext

More from Inverse Trigonometric Functions

  • Match List I with List II and select the correct answer using the code given below the lists: List IP. (y21​(cot(sin−1y)+tan(sin−1y)cos(tan−1y)+ysin(tan−1y)​)2+y4)1/2…2013 · MCQ
  • If 0<x<1, then 1+x2​[{xcos(cot−1x)+sin(cot−1x)}2−1]1/2=2008 · MCQ
  • Let F(x) be an indefinite integral of sin2x. Statement 1 : The function F(x) satisfies F(x+π) = F(x) for all real x. Statement 2 : sin2(x+π)=sin2x for all real x.2007 · MCQ
  • The total number of real solutions of the equation θ=tan−1(2tanθ)−21​sin−1(9+tan2θ6tanθ​) is (Here, the inverse trigonometric functions sin−1x and $\tan^{-1}…2025 · MCQ
  • Considering only the principal values of the inverse trigonometric functions, the value of tan(sin−1(53​)−2cos−1(5​2​)) is2024 · MCQ
  • Let tan−1(x)∈(−2π​,2π​), for x∈R. Then the number of real solutions of the equation 1+cos(2x)​=2​tan−1(tanx) in the set (−23π​,−2π​)∪(−2π​,2π​)∪(2π​,23π​)…2023 · Numerical
  • For any y∈R, let cot−1(y)∈(0,π) and tan−1(y)∈(−2π​,2π​). Then the sum of all the solutions of the equation tan−1(9−y26y​)+cot−1(6y9−y2​)=32π​…2023 · MCQ
  • Considering only the principal values of the inverse trigonometric functions, the value of 23​cos−12+π22​​+41​sin−12+π222​π​+tan−1π2​​ is2022 · Numerical