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posted by Anonymous on Friday, July 9, 2010 at 2:23pm.
Cos(21)cos(9)-sin(21)sin(9)
have you come across this formula ? cos(A+B) = cosAcosB - sinAsinB your expression matches this Cos(21)cos(9)-sin(21)sin(9) = cos(21+9) = cos30 = √3/2
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