# Find angle of a triangle with given sides

1. Apr 26, 2005

### fork

In a triangle ABC, AB=7, BC=8, CA=9, find angle B correct to 3 significant figures.
The answer is 73.4, but i don't know how to calculate it, thanks.

2. Apr 26, 2005

### arildno

Use the cosine rule.

3. Apr 26, 2005

### dextercioby

U'll need a calculator to compute B,i.e.solve the equation

$$\cos B=value$$

and select the first 3 decimals the calculator shows u...

Daniel.

4. Apr 26, 2005

### fork

O, I forgot there is a rule called " cosine rule" in the world, thanks.

5. Apr 26, 2005

### arildno

The world hath many rules, none of which you ought to ignore..