JEE Advanced · Mathematics · 8. Trigonometric Equations
For \(0 < \theta < \frac{\pi}{2}\), the solution(s) of \(\sum_{m=1}^6 \operatorname{cosec}\left[\theta+\frac{(m-1) \pi}{4}\right]\) \(\operatorname{cosec}\left(\theta+\frac{m \pi}{4}\right)=4 \sqrt{2}\) is/are
- A \(\frac{\pi}{4}\)
- B \(\frac{\pi}{6}\)
- C \(\frac{\pi}{12}\)
- D \(\frac{5 \pi}{12}\)
Answer & Solution
Correct Answer
(D) \(\frac{5 \pi}{12}\)
Step-by-step Solution
Detailed explanation
For \(0 < \theta < \frac{\pi}{2}\),
\(\sum_{m=1}^6 \operatorname{cosec}\left[\theta+\frac{(m-1) \pi}{4}\right] \operatorname{cosec}\left(\theta+\frac{m \pi}{4}\right) \)
\( =4 \sqrt{2} \)
\( \Rightarrow \sum_{m=1}^6 \frac{\sin \left[\theta+\frac{m \pi}{4}-\left(\theta+\frac{(m-1) \pi}{4}\right)\right]}{\sin \frac{\pi}{4}\left\{\sin \left(\theta+\frac{(m-1) \pi}{4}\right) \sin \left(\theta+\frac{m \pi}{4}\right)\right\}} \)
\( =4 \sqrt{2}\)
\(\Rightarrow \sum_{m=1}^6 \frac{\cot \left(\theta+\frac{(m-1) \pi}{4}\right)-\cot \left(\theta+\frac{m \pi}{4}\right)}{1 / 2} \)
\( =4 \sqrt{2}\)
\(\Rightarrow \sum_{m=1}^6\left[\cot \left(\theta+\frac{(m-1) \pi}{4}\right)\right. \)
\( \left.-\cot \left(\theta+\frac{m \pi}{4}\right)\right]=4 \)
\( \Rightarrow \cot (\theta)-\cot \left(\theta+\frac{\pi}{4}\right)+\cot \left(\theta+\frac{\pi}{4}\right) \)
\( -\cot \left(\theta+\frac{2 \pi}{4}\right)+\ldots+\cot \left(\theta+\frac{5 \pi}{4}\right) \)
\( -\cot \left(\theta+\frac{6 \pi}{4}\right)=4 \)
\( \Rightarrow \cot \theta-\cot \left(\frac{3 \pi}{2}+\theta\right)=4 \)
\( \Rightarrow \cot \theta+\tan \theta=4 \)
\( \Rightarrow \tan ^2 \theta-4 \tan \theta+1=0 \)
\( \Rightarrow (\tan \theta-2)^2-3=0 \)
\( \Rightarrow(\tan \theta-2+\sqrt{3})(\tan \theta-2-\sqrt{3})=0 \)
\( \Rightarrow \tan \theta=2-\sqrt{3} \text { or } \)
\( \tan \theta=2+\sqrt{3} \)
\( \Rightarrow \theta=\frac{\pi}{12} ; \theta=\frac{5 \pi}{12} \Rightarrow \theta \in\left(0, \frac{\pi}{2}\right)\)
\(\sum_{m=1}^6 \operatorname{cosec}\left[\theta+\frac{(m-1) \pi}{4}\right] \operatorname{cosec}\left(\theta+\frac{m \pi}{4}\right) \)
\( =4 \sqrt{2} \)
\( \Rightarrow \sum_{m=1}^6 \frac{\sin \left[\theta+\frac{m \pi}{4}-\left(\theta+\frac{(m-1) \pi}{4}\right)\right]}{\sin \frac{\pi}{4}\left\{\sin \left(\theta+\frac{(m-1) \pi}{4}\right) \sin \left(\theta+\frac{m \pi}{4}\right)\right\}} \)
\( =4 \sqrt{2}\)
\(\Rightarrow \sum_{m=1}^6 \frac{\cot \left(\theta+\frac{(m-1) \pi}{4}\right)-\cot \left(\theta+\frac{m \pi}{4}\right)}{1 / 2} \)
\( =4 \sqrt{2}\)
\(\Rightarrow \sum_{m=1}^6\left[\cot \left(\theta+\frac{(m-1) \pi}{4}\right)\right. \)
\( \left.-\cot \left(\theta+\frac{m \pi}{4}\right)\right]=4 \)
\( \Rightarrow \cot (\theta)-\cot \left(\theta+\frac{\pi}{4}\right)+\cot \left(\theta+\frac{\pi}{4}\right) \)
\( -\cot \left(\theta+\frac{2 \pi}{4}\right)+\ldots+\cot \left(\theta+\frac{5 \pi}{4}\right) \)
\( -\cot \left(\theta+\frac{6 \pi}{4}\right)=4 \)
\( \Rightarrow \cot \theta-\cot \left(\frac{3 \pi}{2}+\theta\right)=4 \)
\( \Rightarrow \cot \theta+\tan \theta=4 \)
\( \Rightarrow \tan ^2 \theta-4 \tan \theta+1=0 \)
\( \Rightarrow (\tan \theta-2)^2-3=0 \)
\( \Rightarrow(\tan \theta-2+\sqrt{3})(\tan \theta-2-\sqrt{3})=0 \)
\( \Rightarrow \tan \theta=2-\sqrt{3} \text { or } \)
\( \tan \theta=2+\sqrt{3} \)
\( \Rightarrow \theta=\frac{\pi}{12} ; \theta=\frac{5 \pi}{12} \Rightarrow \theta \in\left(0, \frac{\pi}{2}\right)\)
See the Complete Solution
Get step-by-step explanations for this and 2.5 Lakh+ more JEE, NEET & CET questions.
- Unlock all solutions
- Practice the full chapter
- Track accuracy across PYQs
4.8 rated on Google Play · 14,000+ reviews
More questions from Mathematics
- Paragraph:
Let \(U_1\) and \(U_2\) be two urns such that \(U_1\) contains 3 white and 2 red balls and \(U_2\) contains only 1 white ball. A fair coin is tossed. If head appears then 1 ball is drawn at random from \(U_1\) and put into \(U_2\). However, if tail appears then 2 balls are drawn at random from \(U_1\) and put into \(U_2\). Now, 1 ball is drawn at random from \(U_2\).Question:
Given that the drawn ball from \(U_2\) is white, the probability that head appeared on the coin isJEE Advanced 2011 Medium - Consider the given data with frequency distribution Match each entry in List-I to the correct entries in List-II.
The correct option isList-I List-II The mean of the above data is The median of the above data is The mean deviation about the
mean of the above data isThe mean deviation about the
median of the above data isJEE Advanced 2023 Easy - Let denote the number of elements in set Let be a sample space, where each element is equally likely to occur. If and are independent events associated with then the number of ordered pairs such that equalsJEE Advanced 2019 Hard
- Three students \(S_1, S_2\) and \(S_1\) are given a problem to solve. Consider the following events :
\(U:\) At least one of \(S_1, S_2\) and \(S_3\) can solve the problem,
\(V: S_1\) can solve the problem, given that neither \(\mathrm{S}_2\) nor \(\mathrm{S}_3\) can solve the problem,
\(W: S_2\) can solve the problem and \(S_3\) cannot solve the problem,
\(T: S_3\) can solve the problem.
For any event \(E\), let \(P(E)\) denote the probability of \(E\).
If \(P(U)=\frac{1}{2}, P(V)=\frac{1}{10}\) and \(P(W)=\frac{1}{12}\), then \(P(T)\) is equal toJEE Advanced 2025 Medium - The trace of a square matrix is defined to be the sum of its diagonal entries. If is a matrix such that the trace of is and the trace of is then the value of the determinant of is_____JEE Advanced 2020 Hard
- Let \(\mathbf{a}=\hat{\mathbf{i}}+\hat{\mathbf{j}}+\hat{\mathbf{k}}, \mathbf{b}=\hat{\mathbf{i}}-\hat{\mathbf{j}}+\hat{\mathbf{k}}\) and \(\mathbf{c}=\hat{\mathbf{i}}-\hat{\mathbf{j}}-\hat{\mathbf{k}}\) be three vectors. A vector \(\mathbf{v}\) in the plane of \(\mathbf{a}\) and \(\mathbf{b}\) whose projection of \(\mathbf{c}\) is \(\frac{1}{\sqrt{3}}\), is given byJEE Advanced 2011 Medium
More PYQs from JEE Advanced
- A normal with slope \(\frac{1}{\sqrt{6}}\) is drawn from the point \((0,-\alpha)\) to the parabola \(x^2=-4 a y\), where \(a>0\). Let \(L\) be the line passing through \((0,-\alpha)\) and parallel to the directrix of the parabola. Suppose that \(L\) intersects the parabola at two points \(A\) and \(B\). Let \(r\) denote the length of the latus rectum and \(s\) denote the square of the length of the line segment \(A B\). If \(r: s=1: 16\), then the value of 24ais ________.JEE Advanced 2024 Medium
- The heating of \(\mathrm{NH}_4 \mathrm{NO}_2\) at \(60-70^{\circ} \mathrm{C}\) and \(\mathrm{NH}_4 \mathrm{NO}_3\) at \(200-250^{\circ} \mathrm{C}\) is associated with the formation of nitrogen containing compounds \(\mathbf{X}\) and \(\mathbf{Y}\), respectively. \(\mathbf{X}\) and \(\mathbf{Y}\), respectively, areJEE Advanced 2025 Easy
- A sphere is rolling without slipping on a fixed horizontal plane surface. In the figure, \(A\) is the point of contact. \(B\) is the centre of the sphere and \(C\) is its topmost point. Then,
JEE Advanced 2009 Medium - Consider the parabola \(y^2=8 x\). Let \(\Delta_1\) be the area of the triangle formed by the end points of its latusrectum and the point \(P\left(\frac{1}{2}, 2\right)\) on the parabola and \(\Delta_2\) be the area of the triangle formed by drawing tangents at \(P\) and at the end points of the latusrectum. Then, \(\frac{\Delta_1}{\Delta_2}\) isJEE Advanced 2011 Hard
- Paragraph:
\(\mathbf{P}_{17 \text { - } 19}\) : Paragraph for Questions Nos. 17 to 19 Two discs \(A\) and \(B\) are mounted coaxially on a vertical axle. The discs have moments of inertia I and 2I, respectively about the common axis. Disc \(A\) is imparted an initial angular velocity \(2 \omega\) using the entire potential energy of a spring compressed by a distance \(x_1\). Disc \(B\) is imparted an angular velocity \(\omega\) by a spring having the same spring constant and compressed by a distance \(x_2\). Both the discs rotate in the clockwise direction.Question:
The ratio \(\frac{x_1}{x_2}\) isJEE Advanced 2007 Easy - For every pair of continuous functions such that then the correct statement (s) is (are)JEE Advanced 2014 Hard