Law of Cosines: Do I have to learn all three?

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anonmeans
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Law of Cosines: Do I have to learn all three?

Postby anonmeans » Fri Feb 06, 2009 3:21 pm

My book shows three rules for the Law of Cosines:

a2 = b2 + c2 - 2bc cos A
b2 = a2 + c2 - 2ac cos B
c2 = a2 + b2 - 2ab cos C

Don't these all really mean the same thing? "To find (the square of) the length opposite an angle, square the other two sides, and subtract two times the two lengths times the cosine of that angle."

Thank you.

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stapel_eliz
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Postby stapel_eliz » Fri Feb 06, 2009 4:19 pm

anonmeans wrote:Don't these all really mean the same thing? "To find (the square of) the length opposite an angle, square the other two sides, and subtract two times the two lengths times the cosine of that angle."

Yes, all three forms do mean the exact same thing, and you've stated the meaning correctly. And, by understanding that meaning, you won't have to depend on a graphic being labelled with the "right" letters, because you'll be able to extract the necessary relationship yourself.

Good job! :D

Eliz.

anonmeans
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Joined: Sat Jan 24, 2009 7:18 pm
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Re: Law of Cosines: Do I have to learn all three?

Postby anonmeans » Mon Feb 09, 2009 6:42 pm

Thanks. That's a lot easier. 8-)


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