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Prove that

Posted July 19th, 2009 by alpha
in

b cosB +c cos C= a cos (B-C)

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answer

On July 19th, 2009 Structure says:
$$a=2R\sin A$$
$$2R \sin B \cos B+\2R\sin C \cos C=R( \sin 2B+\sin 2C)= 2R\sin (B+C)\cos (B-C)=2R\sin A \cos (B-C)=a\cos (B-C)$$

~~Thank You~~

On July 20th, 2009 alpha says:

-------------------x----------------------------
``Old theorems never die; they turn into definitions.''
----E. Hewitt

''ALPHA" =)

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