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In a DeltaA B C prove that cos(A+B)+cos...

In a `DeltaA B C` prove that `cos(A+B)+cos C=0.`

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△ABC.A+B+C=`180^o`
A+B=`180^o` −C
cos(A+B)+cosC
=cos(`180^o`−C)+cosC=−cosC+cosC
=0
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