# Homework Help: Law of cosine

1. Jan 28, 2007

### disregardthat

1. The problem statement, all variables and given/known data

In triangle ABC where you only know the sides: a, b and c I must find angle B.
a=8
b=6
c=12

2. Relevant equations

Law of cosines: c^2 = a^2 + b^2 -2ab*cos(C)
When angle C is at the opposite of side c, (same for a and b)

3. The attempt at a solution

12^2 = 6^2 + 8^2 -2*6*8*cos(C)

144 = 36+64-96cos(C)
44=-96cos(C)
-(44/96) = cos(C)
- (11/24) = cos(C)

(I know the answer is supposed to be 117.4)
And cos(117.4)=-(11/24)

The problem is:
How do I find the angle? The explanation says: "Use calculator (degree mode)"
How do I do that?

I have an TI-84 Plus

2. Jan 28, 2007

### Dick

Look for 'arccos' or 'cos^(-1)'.

3. Jan 28, 2007

### turdferguson

Are you sure about that? Check your formula again. The largest angle must be opposite the largest side, and side b is not the largest side

4. Jan 29, 2007

### disregardthat

Thanks, I found it, and I filled in the number, and I got the answer.

5. Jan 29, 2007

### disregardthat

Thanks, I found it, and I filled in the number, and I got the answer.