Ejercicio 11. Demostrar: $\sin\theta+\sin2\theta+\cdots+\sin n\theta= \frac{\sin\frac{1}{2}\left(n+1\right)\theta \sin\frac{1}{2} n\theta}{\sin\frac{\theta}{2}}$
Demostración:
Para:
$n=1$, tenemos
$\sin 1\cdot \theta = \frac{\sin\frac{1}{2}\left(1+1\right)\theta \sin\frac{1}{2} 1 \cdot \theta}{\sin\frac{\theta}{2}}$
$\sin 1\cdot \theta = \sin\frac{1}{2}\left(2\right)\theta$
$\sin \theta=\sin \theta$ $\checkmark$
Aunque no se pide revisamos para $n=2$,
$\sin 1\cdot \theta+\sin 2\cdot \theta = \sin\theta+\sin 2\theta$
$ =\sin \theta+2\sin\theta \cos\theta$
$=\left(1+2\cos \theta\right)\sin \theta$
$=\left(3-2+2\cos \theta\right)\sin \theta$
$=\left[3-2\left(1-\cos \theta\right)\right]\sin \theta$
$=\left[3-4\frac{\left(1-\cos \theta\right)}{2}\right]\sin \theta$
$=\left[3-4\sin^2\frac{\theta}{2}\right]\sin \theta$
$ =\frac{\sin\frac{\theta}{2}\left[3-4\sin^2\frac{\theta}{2}\right]\sin \theta}{\sin \frac{\theta}{2}}$
$ =\frac{\left(3\sin\frac{\theta}{2}-4\sin^3\frac{\theta}{2}\right)\sin\theta}{\sin \frac{\theta}{2}}$
$ =\frac{\sin\frac{3\theta}{2} \sin \theta}{\sin \frac{\theta}{2}}.$ $\checkmark$
HI $\sin\theta+\sin2\theta+\cdots+\sin k\theta= \frac{\sin\frac{1}{2}\left(k+1\right)\theta \sin\frac{1}{2} k\theta}{\sin\frac{\theta}{2}}.$ $\checkmark$
TI $\sin\theta+\sin2\theta+\cdots+\sin k^*\theta= \frac{\sin\frac{1}{2}\left(k^*+1\right)\theta \sin\frac{1}{2} k^*\theta}{\sin\frac{\theta}{2}}.$
Prueba:
$\sin\theta+\sin2\theta+\cdots+\sin k^*\theta=\sin\theta+\sin2\theta+\cdots+\sin k\theta+\sin k^*\theta$
$= \frac{\sin\frac{1}{2}\left(k+1\right)\theta \sin\frac{1}{2} k\theta}{\sin\frac{\theta}{2}}+\sin\left( k+1\right)\theta$
$= \frac{\sin\frac{1}{2}\left(k+1\right)\theta \sin\frac{1}{2} k\theta}{\sin\frac{\theta}{2}}+\sin2\left(\frac{1}{2}\left( k+1\right)\right)\theta$
$=\frac{\sin\frac{1}{2}\left(k+1\right)\theta \sin\frac{1}{2} k\theta}{\sin\frac{\theta}{2}}+2\sin\frac{1}{2}\left(k+1\right)\theta\cos\frac{1}{2}\left(k+1\right)\theta$
$=\frac{\sin\frac{1}{2}\left(k+1\right)\theta \sin\frac{1}{2} k\theta+2\sin\frac{\theta}{2}\sin\frac{1}{2}\left(k+1\right)\theta\cos\frac{1}{2}\left(k+1\right)\theta}{\sin\frac{\theta}{2}}$
$=\frac{\sin\frac{1}{2}\left(k+1\right)\theta}{\sin\frac{\theta}{2}}\left[\sin\frac{1}{2} k\theta +2\sin\frac{\theta}{2}\cos \frac{\left(k+1\right)\theta}{2} \right]$
$=\frac{\sin\frac{1}{2}\left(k+1\right)\theta}{\sin\frac{\theta}{2}}\left[\sin\frac{1}{2} k\theta +2\sin\frac{\theta}{2}\cos {\left(\frac{k\theta}{2}+\frac{\theta}{2}\right)} \right]$ $=\frac{\sin\frac{1}{2}\left(k+1\right)\theta}{\sin\frac{\theta}{2}}\left[\sin\frac{1}{2} k\theta +2\sin\frac{\theta}{2}\left(\cos \frac{k\theta}{2}\cos\frac{\theta}{2}-\sin\frac{k\theta}{2}\sin\frac{\theta}{2}\right)\right]$
$=\frac{\sin\frac{1}{2}\left(k+1\right)\theta}{\sin\frac{\theta}{2}}\left[\sin\frac{1}{2} k\theta +2\sin\frac{\theta}{2}\cos \frac{k\theta}{2}\cos\frac{\theta}{2}-2\sin\frac{\theta}{2}\sin\frac{k\theta}{2}\sin\frac{\theta}{2}\right]$
$=\frac{\sin\frac{1}{2}\left(k+1\right)\theta}{\sin\frac{\theta}{2}}\left[\sin\frac{1}{2} k\theta\left(1-2\sin^2\frac{\theta}{2}\right) +\cos \frac{k\theta}{2}2\sin\frac{\theta}{2}\cos\frac{\theta}{2}\right]$
$=\frac{\sin\frac{1}{2}\left(k+1\right)\theta}{\sin\frac{\theta}{2}}\left[\sin\frac{1}{2} k\theta\cos\theta +\cos \frac{k\theta}{2}\sin\theta\right]$ $=\frac{\sin\frac{1}{2}\left(k+1\right)\theta}{\sin\frac{\theta}{2}}\sin\left(\frac{k\theta}{2}+\theta\right)$
$=\frac{\sin\frac{1}{2}\left(k+1\right)\theta}{\sin\frac{\theta}{2}}\sin\left(\frac{k\theta+2\theta}{2}\right)$
$=\frac{\sin\frac{1}{2}\left(k+1\right)\theta\sin\left(\frac{k\theta+2\theta}{2}\right)}{\sin\frac{\theta}{2}}$
$=\frac{\sin\frac{1}{2}\left(k+1\right)\theta\sin\left(\frac{\left(k+2\right)\theta}{2}\right)}{\sin\frac{\theta}{2}}$
$=\frac{\sin\frac{1}{2}\left(k+1\right)\theta\sin\left[\frac{\left(\left(k+1\right)+1\right)\theta}{2}\right]}{\sin\frac{\theta}{2}}$
$=\frac{\sin\frac{1}{2}k^*\theta\sin\left[\frac{\left(k^*+1\right)\theta}{2}\right]}{\sin\frac{\theta}{2}}.$ $\checkmark$