Can you prove this trigonometric identity?

  • MHB
  • Thread starter anemone
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In summary, there are multiple methods for proving a trigonometric identity, including algebraic manipulation, using trigonometric identities and properties, and geometric proofs. It is not necessary to memorize all identities, but having a basic understanding can be helpful. While a calculator can be used to check work, it is not recommended for proving identities. A trigonometric identity is considered true if it holds for all values of the variables and can be proven mathematically. To prove an identity, both sides of the equation must be shown to be equal by using both sides in the proof.
  • #1
anemone
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Prove that $\dfrac{1+\sin 6^{\circ}+\cos 12^{\circ}}{\cos 6^{\circ}+\sin 12^{\circ}}=\sqrt{3} $.

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  • #2
Congratulations to the following members for their correct solutions::)

1. laura123
2. kaliprasad

I'll show both solutions since they are different in the method of approaching.(Yes)

Here's laura123's solution:
Since $\cos 36°=\dfrac{\sqrt{5}+1}{4}$ and $\sin 18°=\dfrac{\sqrt{5}-1}{4}$ it follows:
$\cos 36°-\sin 18°=\dfrac{1}{2}$
$\cos (30°+6°)-\sin (30°-12°)=\dfrac{1}{2}$
$\cos 30°\cos 6°-\sin 30°\sin 6°-\sin 30°\cos 12°+\cos 30°\sin 12°=\dfrac{1}{2}$
$\dfrac{\sqrt{3}}{2}\cos 6°-\dfrac{1}{2}\sin 6°-\dfrac{1}{2}\cos 12°+\dfrac{\sqrt{3}}{2}\sin 12°=\dfrac{1}{2}$
$\sqrt{3}\cos 6°-\sin 6°-\cos 12°+\sqrt{3}\sin 12°=1$
$\sqrt{3}\cos 6°+\sqrt{3}\sin 12°=1+\sin 6°+\cos 12°$
$\sqrt{3}(\cos 6°+\sin 12°)=1+\sin 6°+\cos 12°$
$\dfrac{\sqrt{3}(\cos 6°+\sin 12°)}{\cos 6°+\sin 12°}=\dfrac{1+\sin 6°+\cos 12°}{\cos 6°+\sin 12°}$
$\sqrt{3}=\dfrac{1+\sin 6°+\cos 12°}{\cos 6°+\sin 12°}$.

Here's kaliprasad's solution:
First, note that

$\cos(36^\circ) - \cos(72^\circ)$
= $2 * \sin(18^\circ) * \sin(54^\circ)$
= $\dfrac{1}{2} * \dfrac{\sin(2*18)^\circ * \sin(2*54)^\circ }{\cos(18)^\circ * \cos(54)^\circ}$
= $\dfrac{1}{2} * \dfrac{\sin(36^\circ) * \sin(108^\circ)}{ (\sin(108^\circ) * \sin(36^\circ)})$
= $\dfrac{1}{2}$

We then have
$1+ \sin\ 6^\circ + \cos \ 12^\circ$
= $2(\dfrac{1}{2} +\dfrac{1}{2} \sin\ 6^\circ + \dfrac{1}{2}\cos \ 12^\circ)$
= $2(\dfrac{1}{2} +\cos \ 60^\circ \sin\ 6^\circ + \cos \ 60^\circ\cos \ 12^\circ)$
= $2 ( \dfrac{1}{2}- \sin (60-6)^\circ + \sin 60^\circ \cos 6^\circ + \cos 72^\circ + \sin 60^\circ \sin 12^\circ )$
= $2 ( \dfrac{1}{2}- \sin\ 54^\circ + \sin 60^\circ \cos 6^\circ + \cos 72^\circ + \sin 60^\circ \sin 12^\circ )$
= $2 ( \sin 60^\circ (\cos 6^\circ + \sin 12^\circ ) + \dfrac{1}{2} + \cos 72^\circ - \cos 36^\circ )$
=$2 ( \sin 60^\circ (\cos 6^\circ + \sin 12^\circ ) + \dfrac{1}{2} -( \cos 36^\circ-\cos 72^\circ )$
=$2 ( \sin 60^\circ (\cos 6^\circ + \sin 12^\circ ) + \dfrac{1}{2} -\dfrac{1}{2}$
= $2 \sin 60^\circ ( \cos 6^\circ + \sin 12^\circ )$
= $\dfrac{ 1 + \sin 6^\circ + \cos 12^\circ }{\cos 6^\circ + \sin 12^\circ } = 2 \sin 60^\circ = \sqrt(3)$
 

FAQ: Can you prove this trigonometric identity?

1. How can you prove a trigonometric identity?

There are several methods for proving a trigonometric identity, such as using algebraic manipulation, using trigonometric identities and properties, or using geometric proofs.

2. Is it necessary to memorize all trigonometric identities to prove them?

No, it is not necessary to memorize all trigonometric identities. However, it is helpful to have a basic understanding of the common identities and properties, and to be able to use them in your proof.

3. Can you use a calculator to prove a trigonometric identity?

While a calculator can be helpful in checking your work, it is not recommended to use a calculator to prove a trigonometric identity. This is because the use of a calculator does not demonstrate a thorough understanding of the concept and can lead to errors.

4. How do you know if a trigonometric identity is true?

A trigonometric identity is considered to be true if it holds true for all values of the variables involved, and if it can be proven using mathematical methods.

5. Can you prove a trigonometric identity using only one side of the equation?

No, in order to prove a trigonometric identity, you must show that both sides of the equation are equal. This means using both sides of the equation in your proof and manipulating them until they are equal.

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