Prove that Cos2A+Cos2B-cos2C=1-4sinAsinBsinC
Given that A+B+C=pi
Last edited by matsci0000; Jul 14th 2009 at 08:19 AM.
Reason: inadequate information
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=-2cosC cos(A-B)-2cos^2C+1 [ cos(A+B)=cos(pie-C)= -cosC]
=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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