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August 31, 2015
Posted by **Kewal** on Friday, December 30, 2011 at 4:52am.

sin(n+1)Asin(n+2)A + cos(n+1)Acos(n+2)A = cosA

- Trigonometry -
**drwls**, Friday, December 30, 2011 at 9:31amUse the same method as my other proof, but let

(n+1)A = x and

(n+2)A = y

y-x = A

sin(n+1)Asin(n+2)A+cos(n+1)Acos(n+2)A

= sinxsiny + cosxcosy

= cos(y-x) = cosA