Prove that Cos2A+Cos2B-cos2C=1-4sinAsinBsinC Given that A+B+C=pi
Last edited by matsci0000; Jul 14th 2009 at 07:19 AM. Reason: inadequate information
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(cos2A+cos2B)+cos2C =2cos(A+B)cos(A-B)-cos2C =-2cosC cos(A-B)-2cos^2C+1 [ cos(A+B)=cos(pie-C)= -cosC] =1-2cosC[cos(A-B)-cosC] =1-2cosC[cos(A+B) -cos(A-B)] =1-2cosC [2sinA sinB] How to get the term sinC from 2cosC?
That identity is false. It's actually provided The solution is quite simple: In order to have we require
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