# B Trigonometry Identity Question

Can someone please tell me how sin(180 - x) = sin x?

Here my attempt:

sin (180 - x) = sin 180 . cos x - cos 180 . sin x

Next? I have no idea...

#### phinds

Gold Member
Can someone please tell me how sin(180 - x) = sin x?

Here my attempt:

sin (180 - x) = sin 180 . cos x - cos 180 . sin x

Next? I have no idea...
I suggest that you draw the XY axis, draw show zero degrees, X degrees, and 180-X degrees. Or do you mean that you need to show it algebraically?

I suggest that you draw the XY axis, draw show zero degrees, X degrees, and 180-X degrees. Or do you mean that you need to show it algebraically?
Yes, I need it algebraically.

#### fresh_42

Mentor
2018 Award
Yes, I need it algebraically.
What are $\sin 180°$ and $\cos 180°$?

#### phinds

Gold Member
What are $\sin 180°$ and $\cos 180°$?

#### jedishrfu

Mentor
You should know the values of cosine for common angles. Specifically for the angles of 0, 30, 45,60,90,120,135, 150 and 180 degrees at least and their equivalents in radian measure otherwise you will never solve these kinds of problems.

Did you learn about the unit circle?

https://en.m.wikipedia.org/wiki/Unit_circle

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What are $\sin 180°$ and $\cos 180°$?
I only know the value of sin x and cos x in the range from 0 degree to 90 degree.

I don't know the value of sin x and cos x greater than 90 degree.

#### jedishrfu

Mentor
I only know the value of sin x and cos x in the range from 0 degree to 90 degree.

I don't know the value of sin x and cos x greater than 90 degree.
Then find out. Look at my unit circle reference. In truth you should know them for 360 degrees and beyond as that in itself is also important to understand the periodicity of the functions.

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#### fresh_42

Mentor
2018 Award
Or calculate $\sin (90°+90°)$ by the formulas you already used.