The value of the determinant $$ \left|\begin{array}{ccc} \sin \theta & \cos \theta & \sin 2 \theta \\ \sin [\theta+(2 \pi / 3)] & \cos [\theta+(2 \pi / 3)] & \sin [2 \theta+(4 \pi / 3)] \\ \sin [\theta-(2 \pi / 3)] & \cos [\theta-(2 \pi / 3)] & \sin [2 \theta-(4 \pi / 3)] \end{array}\right| $$ is (a) 0 (b) \(2 \sin \theta\) \(1-\sin 2 \theta \quad\) (d) \(-2 \cos \theta\)

Short Answer

Expert verified
The value of the determinant is not equal to any of the given options.

Step by step solution

01

Rewrite trigonometric functions using identities

We can rewrite the trigonometric functions in the second and third row using the identities \(\sin(a + b)\) and \(\cos(a + b)\). The given determinant is: $$ \left|\begin{array}{ccc} \sin \theta & \cos \theta & \sin 2 \theta \\\ \sin [\theta+(2 \pi / 3)] & \cos [\theta+(2 \pi / 3)] & \sin [2 \theta+(4 \pi / 3)] \\\ \sin [\theta-(2 \pi / 3)] & \cos [\theta-(2 \pi / 3)] & \sin [2 \theta-(4 \pi / 3)] \end{array}\right| $$ After applying the identities, we get: $$ \left|\begin{array}{ccc} \sin \theta & \cos \theta & \sin 2 \theta \\\ \sin \theta \cos(2\pi/3) + \cos \theta \sin(2\pi/3) & \cos \theta \cos(2\pi/3) - \sin \theta \sin(2\pi/3) & \sin 2 \theta \cos(4\pi/3) + \cos 2 \theta \sin(4\pi/3) \\ \sin \theta \cos(-2\pi/3) + \cos \theta \sin(-2\pi/3) & \cos \theta \cos(-2\pi/3) - \sin \theta \sin(-2\pi/3) & \sin 2 \theta \cos(-4\pi/3) + \cos 2 \theta \sin(-4\pi/3) \end{array}\right| $$
02

Simplify the determinant

Now, we simplify the determinant by evaluating the trigonometric functions at the given angles: $$ \left|\begin{array}{ccc} \sin \theta & \cos \theta & \sin 2 \theta \\\ -\frac{1}{2}\sin \theta + \frac{\sqrt{3}}{2}\cos \theta & -\frac{\sqrt{3}}{2}\sin \theta -\frac{1}{2}\cos\theta & -\sin 2 \theta \\ \frac{1}{2}\sin \theta - \frac{\sqrt{3}}{2}\cos \theta & \frac{\sqrt{3}}{2}\sin \theta - \frac{1}{2}\cos\theta & \sin 2 \theta \end{array}\right| $$
03

Expand the determinant

We will expand the determinant along the first row: $$ \begin{aligned} &=\sin \theta \cdot \left|\begin{array}{cc} -\frac{\sqrt{3}}{2}\sin \theta -\frac{1}{2}\cos\theta & -\sin 2 \theta \\ \frac{\sqrt{3}}{2}\sin \theta - \frac{1}{2}\cos\theta & \sin 2 \theta \end{array}\right| - \cos \theta \cdot \left|\begin{array}{cc} -\frac{1}{2}\sin \theta + \frac{\sqrt{3}}{2}\cos \theta & -\sin 2 \theta \\ \frac{1}{2}\sin \theta - \frac{\sqrt{3}}{2}\cos \theta & \sin 2 \theta \end{array}\right| + \sin 2 \theta \cdot \left|\begin{array}{cc} -\frac{1}{2}\sin \theta + \frac{\sqrt{3}}{2}\cos \theta & -\frac{\sqrt{3}}{2}\sin \theta -\frac{1}{2}\cos\theta \\ \frac{1}{2}\sin \theta - \frac{\sqrt{3}}{2}\cos \theta & \frac{\sqrt{3}}{2}\sin \theta - \frac{1}{2}\cos\theta \end{array}\right| \end{aligned} $$
04

Compute the values of the smaller determinants

Compute the values of the smaller determinants: $$ \begin{aligned} &=\sin \theta \cdot [-\frac{3}{4}\sin^2 \theta - \frac{1}{4}\cos^2\theta -\frac{\sqrt{3}}{2}\sin^2 2 \theta] - \cos \theta \cdot [-\frac{3}{4}\cos^2 \theta - \frac{1}{4}\sin^2\theta + \frac{\sqrt{3}}{2}\sin 2 \theta \cos 2 \theta] \\ + & \quad \sin 2 \theta \cdot[-\frac{3}{4}\sin \theta \cos \theta + \frac{1}{4}\sin \theta \cos\theta +\frac{\sqrt{3}}{2}\sin^2\theta - \frac{\sqrt{3}}{2}\cos^2\theta] \end{aligned} $$
05

Simplify and combine like terms

Simplify the expression and combine like terms: $$ \begin{aligned} &=-\frac{1}{2}\sin \theta - \frac{\sqrt{3}}{2}\sin \theta \cos \theta - \frac{\sqrt{3}}{2}\sin 2 \theta \cos 2 \theta + \frac{1}{2} \sin 2 \theta \cos \theta + \frac{\sqrt{3}}{2} \sin^2 \theta \cos \theta \\ \end{aligned} $$ Looking at the given options, none exactly matches the obtained expression. However, we can notice that our simplified expression equals 0 when \(\sin \theta = 0\) or when \(\cos \theta = 0\). This might suggest some errors in the calculation or that the correct choice is (a) 0. However, without more context, we cannot be certain.

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Most popular questions from this chapter

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