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JEE Advanced 2022Paper 1
Exam
JEE Advanced
Year
2022
Paper
Paper 1
Subject
Mathematics
Chapter
Trigonometric Equations

JEE Advanced 2022 · Paper 1 · Mathematics · Trigonometric Equations

Let |M| denote the determinant of a square matrix M . Let g: [0,...

Let ∣M∣\left|M\right| denote the determinant of a square matrix MM. Let g:[0,π2]→Rg: \left[0, \dfrac{\pi}{2}\right] \to \mathbb{R} be the function defined by g(θ)=f(θ)−1+f(π2−θ)−1g\left(\theta\right) = \sqrt{f\left(\theta\right) - 1} + \sqrt{f\left(\dfrac{\pi}{2} - \theta\right) - 1} where f\left(\theta\right) = \dfrac{1}{2}\left|\begin{cases}1 & sin\theta & 1 \\ - sin\theta & 1 & sin\theta \\ - 1 & - sin\theta & 1\end{cases}\right| + \left|\begin{cases}sin\pi & \cos \left(\theta + \dfrac{\pi}{4}\right) & \tan \left(\theta - \dfrac{\pi}{4}\right) \\ \sin \left(\theta - \dfrac{\pi}{4}\right) & - cos\dfrac{\pi}{2} & \log_{e}\left(\dfrac{4}{\pi}\right) \\ cot\left(\theta + \dfrac{\pi}{4}\right) & \log_{e}\left(\dfrac{\pi}{4}\right) & tan\pi\end{cases}\right|
Let p(x)p\left(x\right) be a quadratic polynomial whose roots are the maximum and minimum values of the function g(θ)g\left(\theta\right), and p(2)=2−2p\left(2\right) = 2 - \sqrt{2}. Then, which of the following is/are TRUE ?

Options

  1. AP(3+24)<0P\left(\dfrac{3 + \sqrt{2}}{4}\right) < 0
  2. BP(1+324)>0P\left(\dfrac{1 + 3\sqrt{2}}{4}\right) > 0
  3. CP(52−14)>0P\left(\dfrac{5\sqrt{2} - 1}{4}\right) > 0
  4. DP(5−24)<0P\left(\dfrac{5 - \sqrt{2}}{4}\right) < 0

Correct answer

A. P(3+24)<0P\left(\dfrac{3 + \sqrt{2}}{4}\right) < 0

Step-by-step solution

Given,

f\left(\theta\right) = \dfrac{1}{2}\left|\begin{cases}1 & sin\theta & 1 \\ - sin\theta & 1 & sin\theta \\ - 1 & - sin\theta & 1\end{cases}\right| + \left|\begin{cases}sin\pi & \cos \left(\theta + \dfrac{\pi}{4}\right) & \tan \left(\theta - \dfrac{\pi}{4}\right) \\ \sin \left(\theta - \dfrac{\pi}{4}\right) & - cos\dfrac{\pi}{2} & \log_{e}\left(\dfrac{4}{\pi}\right) \\ \cot \left(\theta + \dfrac{\pi}{4}\right) & \log_{e}\left(\dfrac{\pi}{4}\right) & tan \pi\end{cases}\right|

⇒ f\left(\theta\right) = \dfrac{1}{2}\left|\begin{cases}1 & sin\theta & 1 \\ - sin\theta & 1 & sin\theta \\ - 1 & - sin\theta & 1\end{cases}\right| + \left|\begin{cases}0 & \cos \left(\theta + \dfrac{\pi}{4}\right) & \tan \left(\theta - \dfrac{\pi}{4}\right) \\ \sin \left(\theta - \dfrac{\pi}{4}\right) & 0 & \log_{e}\left(\dfrac{4}{\pi}\right) \\ - \tan \left(\theta - \dfrac{\pi}{4}\right) & - \log_{e}\left(\dfrac{4}{\pi}\right) & 0\end{cases}\right|

Here we used cos⁡(θ+π4)=−sin⁡(θ−π4)\cos \left(\theta + \dfrac{\pi}{4}\right) = - \sin \left(\theta - \dfrac{\pi}{4}\right)

And tan⁡(θ−π4)=−cot⁡(θ+π4)\tan \left(\theta - \dfrac{\pi}{4}\right) = - \cot \left(\theta + \dfrac{\pi}{4}\right)

And log⁡e(4π)=−log⁡e(π4)\log_{e}\left(\dfrac{4}{\pi}\right) = - \log_{e}\left(\dfrac{\pi}{4}\right)

Also sinπ=−cosπ2=tanπ=0sin\pi = - cos\dfrac{\pi}{2} = tan\pi = 0

So, ⇒ f\left(\theta\right) = \dfrac{1}{2}\left|\begin{cases}1 & sin\theta & 1 \\ - sin\theta & 1 & sin\theta \\ - 1 & - sin\theta & 1\end{cases}\right| + \left(\text{skew symmetric}\right)

⇒f(θ)=1+sin⁡2θ⇒ f\left(\theta\right) = 1 + \sin^{2}\theta

So, g(θ)=∣sin⁡θ∣+∣cos⁡θ∣g\left(\theta\right) = \left|\sin\theta\right| + \left|\cos\theta\right|

Now maximum and minimum values are 2\sqrt{2} and 11 respectively.

Now quadratic polynomial will be P(x)=a(x−2)(x−1)P\left(x\right) = a\left(x - \sqrt{2}\right)\left(x - 1\right), where a∈R−{0}a \in R - \left\{0\right\},

But given P(2)=2−2P\left(2\right) = 2 - \sqrt{2} , so a=1a = 1.

∴P(x)=(x−2)(x−1)∴ P\left(x\right) = \left(x - \sqrt{2}\right)\left(x - 1\right)

Now solving all options we get,

P(3+24)=(3−324)⋅(2−14)<0P\left(\dfrac{3 + \sqrt{2}}{4}\right) = \left(\dfrac{3 - 3\sqrt{2}}{4}\right) \cdot \left(\dfrac{\sqrt{2} - 1}{4}\right) < 0

P(1+324)=(1−24)⋅(32−34)<0P\left(\dfrac{1 + 3\sqrt{2}}{4}\right) = \left(\dfrac{1 - \sqrt{2}}{4}\right) \cdot \left(\dfrac{3\sqrt{2} - 3}{4}\right) < 0

P(52−14)=(2−14)⋅(52−54)>0P\left(\dfrac{5\sqrt{2} - 1}{4}\right) = \left(\dfrac{\sqrt{2} - 1}{4}\right) \cdot \left(\dfrac{5\sqrt{2} - 5}{4}\right) > 0

P(5−24)=(5−524)(1−24)>0P\left(\dfrac{5 - \sqrt{2}}{4}\right) = \left(\dfrac{5 - 5\sqrt{2}}{4}\right)\left(\dfrac{1 - \sqrt{2}}{4}\right) > 0

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