Cosine Rule Help Please! (1 Viewer)

[Farscape]

When using co-sine rule to find and angle how do I know which numbers (lengths) on the triangle are a, b, c in the equation? I always get them mixed up by putting the wrong numbers with the wrong letter.

Any help would be much appreciated

 

[mc88]

This is how I learn't the rule...

c²=a²+b²-2abcosC

Now, all you need to do is make sure that the angle you are using corresponds with the side that you are using for that same letter. So if you use a a certain angle for C, the side opposite that will be the one you substitute in for the c.

EDIT: Sorry, you said for finding the angle. It's the exact same though - just rearrange that formula to make cosC the subject and make sure the value that you substitute in for c is the side opposite the angle you are trying to find.

 

[Forbidden.]

a² = b² + c² - 2bc cosA
2bc cosA = b² + c² - a²
cosA = b² + c² - a² / 2bc