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किसी Delta ABC में सिध्द कीजिये कि: ...

किसी `Delta ABC` में सिध्द कीजिये कि:
`(a) cos 2A + cos 2B + cos 2C =-1- 4 cos A cos B cos C`
`( b) cos 2A + cos 2B-cos 2C=1 -4 sin A sin B cos C`

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