# Homework Help: Compounded Angle Identities

1. Dec 1, 2008

### moe11

1. The problem statement, all variables and given/known data
cos($$\pi$$-x)= -cosX

the formula is cos(A-b) = cosAcosB+sinAsinB
so i sub in the given to get..
cos$$\pi$$cosx + sin$$\pi$$sinX

then where do i go from there? im new to math like this, its a much higher level than what im used to, any help would be very apprieciated. thanks.

2. Relevant equations

3. The attempt at a solution

2. Dec 1, 2008

### rock.freak667

What does cos(pi) and sin(pi) work out to be?

3. Dec 1, 2008

### moe11

sinpi= 0.054803665
cospi= 0.998497149

i dont know where to go from there, i know after the dust settles i need to have -cosx somehow.

4. Dec 1, 2008

### Hurkyl

Staff Emeritus
Those should be values you have memorized....

(And incidentally, you left your calculator in degree mode on accident)

5. Dec 1, 2008

### Mentallic

Are you familiar with radians? $$\pi =180^o$$

6. Dec 2, 2008

### Дьявол

You can use both radians and degrees. If you use the radian mode you will write π = 3.141592 and if you use degree mode you will write п = 180o. Anyway, you will get same value. But these values are so easy to remember (even you don't need to remember it, just draw a circle in coordinate system, and remember the x axis is cos and the y axis is sin). Now turn for 180o from 0o and you will get what?

Regards.