Find the angle between (5i+3j) and (3i?5j).
FURTHER MATHEMATICS
WAEC 2014
Find the angle between (5i+3j) and (3i?5j).
- A. 180°
- B. 90°
- C. 45°
- D. 0°
Correct Answer: B. 90°
Explanation
\(a . b = |a||b|\cos \theta\)
\(\cos \theta = \frac{a . b}{|a||b|}\)
= \( \frac{(5i + 3j).(3i - 5j)}{(\sqrt{5^2 + 3^2})(\sqrt{3^{2} + (-5)^{2}})}\)
= \(\frac{0}{34} = 0\)
\(\theta = \cos^{-1} 0 = 90°\)
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