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January 31, 2015

January 31, 2015

Posted by **Anonymous** on Friday, July 9, 2010 at 2:23pm.

- Trig -
**Reiny**, Friday, July 9, 2010 at 2:25pmhave 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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