prove that if sin(a/2) ≠ 0 then

cosa + cos2a +...+cos(n+1)a = (sin((n+1)a/2)*cos((n+1)a/2)/sin(a/2)

FInd this sum also in the case sin(a/2) = 0

Find a similar formula for sina + sin 2a +...+ sin(n+1)a

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- March 24th 2011, 03:14 PMbeczprove that if sin(a/2) ≠ 0 then...
prove that if sin(a/2) ≠ 0 then

cosa + cos2a +...+cos(n+1)a = (sin((n+1)a/2)*cos((n+1)a/2)/sin(a/2)

FInd this sum also in the case sin(a/2) = 0

Find a similar formula for sina + sin 2a +...+ sin(n+1)a - March 30th 2011, 11:16 AMveileen

Multiply it by . When you have sums like this - multiply by , r is ratio. Is signs alternate multiply the sum by . - March 31st 2011, 04:03 AMbecz
Not sure what you mean.

- March 31st 2011, 11:35 AMveileen

"Is signs alternate multiply the sum by ." for example: - April 2nd 2011, 12:30 PMbecz
still not sure what you mean

- April 2nd 2011, 12:31 PMveileen
Okay, what do you think I mean?