WAEC - Further Mathematics (2014 - No. 24)
Find the angle between \((5i + 3j)\) and \((3i - 5j)\).
180°
90°
45°
0°
Giải thích
\(a . b = |a||b|\cos \theta\)
\(\cos \theta = \frac{a . b}{|a||b|}\)
= \( \frac{(5i + 3j).(3i - 5j)}{(\sqrt{5^2 + 3^2})(\sqrt{3^{2} + (-5)^{2}})}\)
= \(\frac{0}{34} = 0\)
\(\theta = \cos^{-1} 0 = 90°\)