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If A+B+C=180^@ then cos2A+cos2B+cos2C=1-...

If `A+B+C=180^@` then `cos2A+cos2B+cos2C=1-4cosAcosBcosC` 2. If `A+B+C=0` then `cosA+cosB+cosC=-1+4cosA/2cosB/2cosC/2`

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