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JEE Advanced 2022 · Determinants

Let ∣M∣|M| denote the determinant of a square matrix MM. Let g:[0,π2]→Rg:\left[0,\frac{\pi}{2}\right]\rightarrow\mathbb{R} be the function defined by where g(θ)=f(θ)−1+f(π2−θ)−1g(\theta)=\sqrt{f(\theta)-1}+\sqrt{f\left(\frac{\pi}{2}-\theta\right)-1} f(θ)=12∣1sin⁡θ1−sin⁡θ1sin⁡θ−1−sin⁡θ1∣+∣sin⁡πcos⁡(θ+π4)tan⁡(θ−π4)sin⁡(θ−π4)−cos⁡π2log⁡e(4π)cot⁡(θ+π4)log⁡e(π4)tan⁡π∣f(\theta)=\frac{1}{2}\begin{vmatrix}1 & \sin\theta & 1 \\ -\sin\theta & 1 & \sin\theta \\ -1 & -\sin\theta & 1\end{vmatrix}+\begin{vmatrix}\sin\pi & \cos\left(\theta+\frac{\pi}{4}\right) & \tan\left(\theta-\frac{\pi}{4}\right) \\ \sin\left(\theta-\frac{\pi}{4}\right) & -\cos\frac{\pi}{2} & \log_e\left(\frac{4}{\pi}\right) \\ \cot\left(\theta+\frac{\pi}{4}\right) & \log_e\left(\frac{\pi}{4}\right) & \tan\pi\end{vmatrix} Let p(x)p(x) be a quadratic polynomial whose roots are the maximum and minimum values of the function g(θ)g(\theta), and p(2)=2−2p(2)=2-\sqrt{2}. Then, which of the following is/are TRUE ?

Answers: A. p(3+24)<0p\left(\frac{3+\sqrt{2}}{4}\right)<0 · C. p(52−14)>0p\left(\frac{5\sqrt{2}-1}{4}\right)>0

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g(θ)=f(θ)−1+f(π2−θ)−1g(\theta)=\sqrt{f(\theta)-1}+\sqrt{f\left(\dfrac{\pi}{2}-\theta\right)-1} f(θ)=12det⁡(A)+det⁡(B),θ∈[0,π2]f(\theta)=\dfrac{1}{2}\det(A)+\det(B),\quad \theta\in\left[0,\dfrac{\pi}{2}\right] p(x): roots =max⁡g(θ), min⁡g(θ);p(2)=2−2p(x):\ \text{roots } = \max g(\theta),\ \min g(\theta);\quad p(2)=2-\sqrt{2} Which of (1)–(4) are TRUE?\text{Which of (1)--(4) are TRUE?} Plan: 1. det⁡A,2. det⁡B,3. f=1+sin⁡2θ\text{Plan: } 1.\ \det A,\quad 2.\ \det B,\quad 3.\ f=1+\sin^2\theta 4. g=sin⁡θ+cos⁡θ,5. p from its roots4.\ g=\sin\theta+\cos\theta,\quad 5.\ p \text{ from its roots} A=∣1sin⁡θ1−sin⁡θ1sin⁡θ−1−sin⁡θ1∣A=\begin{vmatrix}1 & \sin\theta & 1 \\ -\sin\theta & 1 & \sin\theta \\ -1 & -\sin\theta & 1\end{vmatrix} det⁡A=(1+sin⁡2θ)−sin⁡θ(0)+(1+sin⁡2θ)=2(1+sin⁡2θ)\det A=(1+\sin^2\theta)-\sin\theta(0)+(1+\sin^2\theta)=2(1+\sin^2\theta) sin⁡ ⁣(θ−π4)=−cos⁡ ⁣(θ+π4)\sin\!\left(\theta-\tfrac\pi4\right)=-\cos\!\left(\theta+\tfrac\pi4\right) tan⁡ ⁣(θ−π4)=−cot⁡ ⁣(θ+π4),ln⁡π4=−ln⁡4π\tan\!\left(\theta-\tfrac\pi4\right)=-\cot\!\left(\theta+\tfrac\pi4\right),\quad \ln\tfrac\pi4=-\ln\tfrac4\pi B=(0c−d−c0Ld−L0),BT=−BB=\begin{pmatrix}0 & c & -d \\ -c & 0 & L \\ d & -L & 0\end{pmatrix},\quad B^{T}=-B odd-order skew-symmetric⇒det⁡B=0\text{odd-order skew-symmetric} \Rightarrow \det B=0 f(θ)=12det⁡A+det⁡B=1+sin⁡2θf(\theta)=\tfrac12\det A+\det B=1+\sin^2\theta g(θ)=sin⁡2θ+sin⁡2 ⁣(π2−θ)=∣sin⁡θ∣+∣cos⁡θ∣g(\theta)=\sqrt{\sin^2\theta}+\sqrt{\sin^2\!\left(\tfrac\pi2-\theta\right)}=|\sin\theta|+|\cos\theta| θ∈[0,π2]⇒g(θ)=sin⁡θ+cos⁡θ\theta\in\left[0,\tfrac\pi2\right]\Rightarrow g(\theta)=\sin\theta+\cos\theta g(θ)=2 sin⁡ ⁣(θ+π4)g(\theta)=\sqrt2\,\sin\!\left(\theta+\tfrac\pi4\right) max⁡g=2,min⁡g=1\max g=\sqrt2,\qquad \min g=1 p(x)=k(x−1)(x−2)p(x)=k(x-1)(x-\sqrt2) p(2)=k(1)(2−2)=2−2⇒k=1p(2)=k(1)(2-\sqrt2)=2-\sqrt2 \Rightarrow k=1 p(x)=(x−1)(x−2)=x2−(1+2)x+2p(x)=(x-1)(x-\sqrt2)=x^2-(1+\sqrt2)x+\sqrt2 p(x)<0  ⟺  1<x<2p(x)<0 \iff 1<x<\sqrt2 (1) x=3+24∈(1,2)⇒p<0 TRUE(1)\ x=\dfrac{3+\sqrt2}{4}\in(1,\sqrt2)\Rightarrow p<0\ \textbf{TRUE} (2) x=1+324∈(1,2)⇒p<0 FALSE(2)\ x=\dfrac{1+3\sqrt2}{4}\in(1,\sqrt2)\Rightarrow p<0\ \textbf{FALSE} (3) x=52−14>2⇒p>0 TRUE(3)\ x=\dfrac{5\sqrt2-1}{4}>\sqrt2\Rightarrow p>0\ \textbf{TRUE} (4) x=5−24<1⇒p>0 FALSE(4)\ x=\dfrac{5-\sqrt2}{4}<1\Rightarrow p>0\ \textbf{FALSE} TRUE statements: (1) and (3)\text{TRUE statements: }(1)\ \text{and}\ (3)

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