# prove that if sin(a/2) ≠ 0 then...

• March 24th 2011, 02:14 PM
becz
prove 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, 10:16 AM
veileen
$cosa + cos2a +...+cos(n+1)a=S$

Multiply it by $sin(\frac{a}{2})$. When you have sums like this - multiply by $sin(\frac{r}{2})$, r is ratio. Is signs alternate multiply the sum by $cos(\frac{r}{2})$.
• March 31st 2011, 03:03 AM
becz
Not sure what you mean.
• March 31st 2011, 10:35 AM
veileen
$cosa + cos2a +...+cos(n+1)a=S \Rightarrow (cosa + cos2a +...+cos(n+1)a)sin(\frac{a}{2})=S\cdot sin(\frac{a}{2})$
$sin a \cdot cos b=\frac{1}{2}(sin (a+b)+sin(a-b))$

"Is signs alternate multiply the sum by $cos(\frac{a}{2})$." for example: $S=cos a - cos 2a + cos 3a - cos 4a+...+(-1)^n cos(n+1)a$
• April 2nd 2011, 11:30 AM
becz
still not sure what you mean
• April 2nd 2011, 11:31 AM
veileen
Okay, what do you think I mean?