# Trigonometrycot(θ) = tan(2θ - 3π) find 0 < θ < 2π

#### karush

##### Well-known member
$\cot{(\theta)}=\tan{(2\theta-3\pi)}$ find $0<\theta<2\pi$

From the periodic Formula $\tan{(\theta+\pi n)}=\tan{\theta}$

thus
$\displaystyle \cot{(\theta)} =\tan{(2\theta)} \Rightarrow \frac{1}{\tan{\theta}} =\tan{(2\theta)}$

there are 6 answers to this, but stuck here

#### MarkFL

Staff member
I would stop at:

$$\displaystyle \cot(\theta)=\tan(2\theta)$$

Then, I would try combining the following:

Co-function identity:

$$\displaystyle \cot(\theta)=\tan\left(\frac{\pi}{2}-\theta \right)$$

Periodicity of tangent function:

$$\displaystyle \tan(\theta)=\tan(\theta+k\pi)$$ where $$\displaystyle k\in\mathbb{Z}$$

This will give you the six roots you desire.

#### karush

##### Well-known member
by this I assume

$\displaystyle 2 \theta =\frac{\pi}{2}-\theta$
so
$\displaystyle \theta=\frac{\pi}{6}$
then for $\displaystyle 0<\theta<2\pi$ using $\tan{(\theta+\pi n)}=\tan{\theta}$ for period
$$\theta = \frac{\pi}{6}, \frac{7\pi}{6}$$
or
$$30^o,210^o$$

but that is only 2 of them

#### MarkFL

Staff member
You would actually have:

$$\displaystyle \theta=\frac{\pi}{2}-2\theta+k\pi$$

See what you get from that.

#### soroban

##### Well-known member
Hello, karush!

$$\cot \theta\:=\:\tan(2\theta-3\pi),\quad 0<\theta<2\pi$$

$$\tan(2\theta - 3\pi) \;=\;\frac{\tan(2\theta) - \tan(3\pi)}{1 + \tan(2\theta)\tan(3\pi)} \;=\;\tan(2\theta)$$

. . . . . . . . . . $$=\;\frac{2\tan\theta}{1-\tan^2\theta}$$

The equation becomes: .$$\frac{1}{\tan\theta} \;=\;\frac{2\tan\theta}{1-\tan^2\theta}$$

. . $$1 - \tan^2\theta \:=\:2\tan^2\theta \quad\Rightarrow\quad 3\tan^2\theta \:=\:1$$

. . $$\tan^2\theta \:=\:\frac{1}{3} \quad\Rightarrow\quad \tan\theta \:=\:\pm\frac{1}{\sqrt{3}}$$

Therefore: .$$\theta \;=\;\frac{\pi}{6},\;\frac{5\pi}{6},\;\frac{7\pi}{6},\;\frac{11\pi}{6}$$