Tentukan nilai eksak dari \(\dfrac{\sin 1958^{\text{o}} \cos 1290^{\text{o}}}{\sin 420^{\text{o}} \cos 788^{\text{o}}}\)
\begin{equation*}
\begin{split}
& \frac{\sin 1958^{\text{o}} \cos 1290^{\text{o}}}{\sin 420^{\text{o}} \cos 788^{\text{o}}} \\\\
& \frac{\sin (360^{\text{o}} \:.\: 5 + 158^{\text{o}}) \cos (360^{\text{o}} \cdot 3 + 210^{\text{o}})}{\sin (360^{\text{o}} + 60^{\text{o}}) \cos (360^{\text{o}} \:.\: 2 + 68^{\text{o}})} \\\\
& \frac{\sin 158^{\text{o}} \cos 210^{\text{o}}}{\sin 60^{\text{o}} \cos 68^{\text{o}}} \\\\
& \frac{\sin (180^{\text{o}} - 22^{\text{o}}) \cos (180^{\text{o}} + 30^{\text{o}})}{\sin 60^{\text{o}} \cos 68^{\text{o}}} \\\\
& \frac{\sin 22^{\text{o}} (-\cos 30^{\text{o}})}{\sin 60^{\text{o}} \cos 68^{\text{o}}} \\\\
& \frac{\sin 22^{\text{o}} (-\cos 30^{\text{o}})}{\sin (90^{\text{o}} - 30^{\text{o}}) \cos (90^{\text{o}} - 22^{\text{o}})} \\\\
& \frac{\sin 22^{\text{o}} (-\cos 30^{\text{o}})}{\cos 30^{\text{o}} \sin 22^{\text{o}}} \\\\
& -1
\end{split}
\end{equation*}