If the real part of the complex number

$\mathrm{z}=\frac{1}{1-\cos \theta+2 \mathrm{i} \sin \theta}$

$=\frac{2 \sin ^{2} \frac{\theta}{2}-2 \mathrm{i} \sin \theta}{(1-\cos \theta)^{2}+4 \sin ^{2} \theta}$

$=\frac{\sin \frac{\theta}{2}-2 \mathrm{i} \cos \frac{\theta}{2}}{4 \sin \frac{\theta}{2}\left(\sin ^{2} \frac{\theta}{2}+4 \cos ^{2} \frac{\theta}{2}\right)}$

$\operatorname{Re}(z)=\frac{1}{2\left(\sin ^{2} \frac{\theta}{2}+4 \cos ^{2} \frac{\theta}{2}\right)}=\frac{1}{5}$

$\sin \frac{2 \theta}{2}+4 \cos ^{2} \frac{\theta}{2}=\frac{5}{2}$

$1-\cos ^{2} \frac{\theta}{2}+4 \cos \frac{\theta}{2}=\frac{5}{2}$

$3 \cos ^{2} \frac{\theta}{2}=\frac{3}{2}$

$\cos ^{2} \frac{\theta}{2}=\frac{1}{2}$

$\frac{\theta}{2}=\mathrm{n} \pi \pm \frac{\pi}{4}$

$\theta=2 \mathrm{n} \pi \pm \frac{\pi}{2}$

$\theta=2 \mathrm{n} \pi \pm \frac{\pi}{2}$

$\theta \in(0, \pi)$

$\theta=\frac{\pi}{2}$

$\int_{0}^{\frac{\pi}{2}} \sin \theta d \theta-[-\cos \theta]_{0}^{\frac{\pi}{2}}$

$=-(0-1)$

$=1$