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Show that if [tex]\cos \left(\theta+30^{\circ}\right)=\frac{1}{2} \sin \theta[/tex], then [tex]\tan \theta=\frac{\sqrt{3}}{2}[/tex].

Sagot :

Alright, let's tackle this problem step-by-step.

Given:
[tex]\[ \cos \left(\theta + 30^\circ\right) = \frac{1}{2} \sin \theta \][/tex]

We will use the angle addition formula for the cosine function:
[tex]\[ \cos \left(\theta + 30^\circ\right) = \cos \theta \cos 30^\circ - \sin \theta \sin 30^\circ \][/tex]

We know from trigonometric identities that:
[tex]\[ \cos 30^\circ = \frac{\sqrt{3}}{2} \quad \text{and} \quad \sin 30^\circ = \frac{1}{2} \][/tex]

Substituting these values into the angle addition formula, we get:
[tex]\[ \cos \left(\theta + 30^\circ\right) = \cos \theta \cdot \frac{\sqrt{3}}{2} - \sin \theta \cdot \frac{1}{2} \][/tex]

Let's substitute this back into the given equation:
[tex]\[ \cos \theta \cdot \frac{\sqrt{3}}{2} - \sin \theta \cdot \frac{1}{2} = \frac{1}{2} \sin \theta \][/tex]

Now, we need to simplify this equation:
[tex]\[ \cos \theta \cdot \frac{\sqrt{3}}{2} - \sin \theta \cdot \frac{1}{2} - \frac{1}{2} \sin \theta = 0 \][/tex]

Combine the [tex]\(\sin \theta\)[/tex] terms:
[tex]\[ \cos \theta \cdot \frac{\sqrt{3}}{2} - \sin \theta \left(\frac{1}{2} + \frac{1}{2}\right) = 0 \][/tex]

This simplifies to:
[tex]\[ \cos \theta \cdot \frac{\sqrt{3}}{2} - \sin \theta \cdot 1 = 0 \][/tex]
or:
[tex]\[ \cos \theta \cdot \frac{\sqrt{3}}{2} = \sin \theta \][/tex]

Rearranging the terms gives:
[tex]\[ \cos \theta \cdot \frac{\sqrt{3}}{2} = \sin \theta \][/tex]

To isolate [tex]\(\tan \theta\)[/tex], divide both sides by [tex]\(\cos \theta\)[/tex]:
[tex]\[ \frac{\sqrt{3}}{2} = \frac{\sin \theta}{\cos \theta} \][/tex]

Recall that [tex]\(\frac{\sin \theta}{\cos \theta} = \tan \theta\)[/tex]:

So, we have:
[tex]\[ \tan \theta = \frac{\sqrt{3}}{2} \][/tex]

Hence, we have shown that:
[tex]\[ \tan \theta = \frac{\sqrt{3}}{2} \][/tex]

This completes the solution.
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