# Find angle of a triangle with given sides

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.

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arildno
Homework Helper
Gold Member
Dearly Missed
Use the cosine rule.

dextercioby
Homework Helper
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.

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

arildno