3. \( \cos 139^{\circ} \)

Did you know that \( \cos 139^{\circ} \) can be found using the unit circle? This angle lies in the second quadrant where cosine values are negative. You can relate \( 139^{\circ} \) to its reference angle of \( 180^{\circ} - 139^{\circ} = 41^{\circ} \). So, \( \cos 139^{\circ} = -\cos 41^{\circ} \). Using a calculator, you can find \( \cos 41^{\circ} \) and determine the exact value! When calculating cosine for angles like \( 139^{\circ} \), be cautious of the signs! Remember that in the second quadrant, cosine values are negative. A common mistake is to apply the positive cosine rule carelessly, so always double-check the quadrant of your angle to ensure you're identifying the sign correctly!