Hello, rcs!

Recall the sum-to-product identity:

. .

Then take the pairs of cosines "from each end":

. .

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- December 4th 2010, 04:07 AM #1

- December 4th 2010, 05:59 AM #2

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- December 4th 2010, 02:19 PM #3

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- December 4th 2010, 03:28 PM #4

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