# Trigonometric Identities

Hello, I have face a deadlock while trying to find the way to evaluate this identities. I'm curious that we need to use the trigonometry sum formula to find the answer of this problem, but I still cannot find the way to finish it, I always go back to the initial problem. Pls help me, so I can answer my curiosity.. Thanks

Note by Leonardo Chandra
6 years, 4 months ago

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You can simplify this to $4sin 13^\circ cos 31^\circ cos 7^\circ$.

You use the following facts to simplify your expression:

$1)$ $sin A + sin B=2sin(\frac{A+B}{2}) cos(\frac{A-B}{2})$

$(2)$ $sin A - sin B=2sin(\frac{A-B}{2}) cos(\frac{A+B}{2})$

$(3)$ $cos A + cos B=2cos(\frac{A+B}{2}) cos(\frac{A-B}{2})$

$(4)$ $cos A - cos B=-2sin(\frac{A+B}{2}) sin(\frac{A-B}{2})$

If you want to know the proofs for these, they are pretty straightforward.

For example:

We can derive, $sin(x+y)+sin(x-y)=2sinxcosy$

Now, let $x+y=A$ and $x-y=B$, and substitute in the above equation.

This will give you the proof for equation $(1)$

- 6 years, 4 months ago

thanks Aditya, now I've just got some knowledge about simplifying the trigonometric identities..:)

- 6 years, 4 months ago

Hint: $53 - 39 = 14 = 25 - 11$, so that suggests a place to start. Likewise, $53 - 25 = 28 = 39 - 11$.

Staff - 6 years, 4 months ago

what is this sir...I cant understand!

- 6 years, 4 months ago

Think about how to apply the hint, esp in light of the sum and product trigonometric formulas that Aditya mentioned.

Since $53 - 39 = 14$, when we combine $\cos 53 + \cos 39$, we will get $\cos \frac{53-39}{2}$ as one of the terms in the product. Since $25 - 11 = 14$, when we combine $- ( sin 25 + \sin 11)$, we will get $\cos \frac{25-11}{2}$ as one of the terms in the product. This allows us to factor out the $\cos 7$ (which you can see in Aditya's answer) and then continue with the remaining terms.

Staff - 6 years, 4 months ago

Aditya your 4th identity is wrong as it should be B-A instead of A-B which is very important as it matters a lot in sin.

- 6 years, 4 months ago

The 4th identity can, alternatively, also have a negative sign, which I supposedly missed while posting my comment. Thanks.

- 6 years, 4 months ago

awh!!

- 4 years, 10 months ago

- 6 years, 4 months ago

don't use a calc...

- 6 years, 4 months ago

Hello..?? We are now in the 21st century... Use calculator or even more sophisticated computing devices like computers installed with math soft wares like mathlab, scilab and python. There are a lot of builtin functions in that three softwares,... one of which you can convert decimal to fraction with higher precision.. you type that in the console, let the software computes...

- 6 years, 4 months ago

You are missing the point. Calculators will only get you an approximate decimal answer. He wants an exact answer in trigonometric form. See Adiya's solution.

Staff - 6 years, 4 months ago