WAEC - Further Mathematics (2018 - No. 18)
If \(\sin\theta = \frac{3}{5}, 0° < \theta < 90°\), evaluate \(\cos(180 - \theta)\).
\(\frac{4}{5}\)
\(\frac{3}{5}\)
\(\frac{-3}{5}\)
\(\frac{-4}{5}\)
설명
Given \(\sin \theta = \frac{3}{5} \implies opp = 3, hyp = 5\)
Using Pythagoras' Theorem, we have \( adj = \sqrt{5^{2} - 3^{2}} = \sqrt{16} = 4\)
\(\therefore \cos \theta = \frac{4}{5}, 0° < \theta < 90°\)
In the quadrant where \(180° - \theta\) lies is the 2nd quadrant and here, only \(\sin \theta = +ve\).
\(\therefore \cos (180 - \theta) = -ve = \frac{-4}{5}\)