Question : Using the identity, $\tan 2 \alpha = \frac{2 \tan \alpha}{1 – \tan ^2 \alpha},$ find the value of $\tan15°,$ correct to three decimal places. [Use $\sqrt{3} = 1.732$]
Option 1: 0.268
Option 2: 0.27
Option 3: 0.267
Option 4: 0.269
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Correct Answer: 0.268
Solution :
$\tan 2 \alpha = \frac{2 \tan \alpha}{1 – \tan^2 \alpha}$
Putting $\alpha = 15°$
$\tan 2 \times 15°= \frac{2 \tan 15°}{1 – \tan^2 15°}$
$⇒\tan 30°=\frac{2 \tan 15°}{1–\tan^2 15°}$
$⇒\frac{1}{\sqrt{3}} = \frac{2 \tan 15°}{1 – \tan ^2 15°}$
$⇒1 – \tan^2 15° = \sqrt{3}\times 2 \tan 15°$
$⇒\tan^2 15°+ \sqrt{3}\times 2 \tan 15° – 1 = 0$
$⇒\tan 15° = \frac{–2\sqrt{3}\pm \sqrt{(2\sqrt{3})^{2} + 4}}{2}$
$⇒\tan 15° = \frac{–2\sqrt{3}\pm \sqrt{12 + 4}}{2}$
$⇒\tan 15° = \frac{–2\sqrt{3}\pm \sqrt{16}}{2}$
$⇒\tan 15° = \frac{–2\sqrt{3}\pm 4}{2}$
$⇒\tan 15° = –\sqrt{3}\pm 2$
$⇒\tan 15° = ( 2 - \sqrt{3}) \text{or} (-2- \sqrt{3})$
Substituting the value of $\sqrt{3} = 1.732$, we get,
$\tan 15° = 0.268 \text{ or, } –3.732$
Since 15° is in the first quadrant, tan theta is always positive.
Hence, the correct answer is 0.268.
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