Question
Find the principal solutions of $\sin \theta=\frac{1}{\sqrt{2}}$.

Answer

As $\sin \frac{\pi}{4}=\frac{1}{\sqrt{2}}$ and $0 \leq \frac{\pi}{4}<2 \pi, \frac{\pi}{4}$ is a principal solution.
By allied angle formula, $\sin \theta=\sin (\pi-\theta)$.
$
\therefore \sin \frac{\pi}{4}=\sin \left(\pi-\frac{\pi}{4}\right)=\sin \frac{3 \pi}{4} \text { and } 0 \leq \frac{3 \pi}{4}<2 \pi
$
$\therefore \frac{3 \pi}{4}$ is also a principal solution.
$\therefore \frac{\pi}{4}$ and $\frac{3 \pi}{4}$ are the principal solutions of $\sin \theta=\frac{1}{\sqrt{2}}$.

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