A+B+C=180° THEN COS2A+COS2B+COS2C=
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use the following formula for the first 2 terms,
CosC + CosD = 2Cos[(C+D)/2]Cos[(C-D)/2]
then cos 2c is cos (2pi – (2A + 2 B))
CosC + CosD = 2Cos[(C+D)/2]Cos[(C-D)/2]
then cos 2c is cos (2pi – (2A + 2 B))
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