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prove that the following is true- cos 2 A+cos 2 B+cos 2 C=1-2cosAcosBcosC

 
prove that the following is true-
cos2A+cos2B+cos2C=1-2cosAcosBcosC

Grade:11

1 Answers

Vikas TU
14149 Points
6 years ago
Take R.H.S,
1-2cosAcosBcosC,
apply 2cosAcosB formulae and split into a+b form .
Then write 1 as (sinA)^2 + (cosA)^2 and the sort out u would get L.H.S

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