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What is the angle between \(a = (3i - 4j)\) and \(b = (6i +...

Further Mathematics
WAEC 2010

What is the angle between \(a = (3i - 4j)\) and \(b = (6i + 4j)\)?

  • A. 13°
  • B. 87°
  • C. 100°
  • D. 110°
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Correct Answer: Option B
Explanation

\(a . b = |a||b| \cos \theta\)

\(a = 3i - 4j; b = 6i + 4j\)

\(18 - 16 = (\sqrt{3^{2} + (-4)^{2}})(\sqrt{6^{2} + 4^{2}}) \cos \theta\)

\(2 = 5\sqrt{52} \cos \theta\)

\(\cos \theta = \frac{2}{5\sqrt{52}} = 0.0555\)

\(\theta = 86.8° \approxeq 87°\)


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