ISRO Mechanical & Aerospace 2009 - Question 78

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If a + b + c = 0 and |a|=3, |b|=5, |c|=7, then the angle between a and b is

  • A. 45°
  • B. 30°
  • C. 90°
  • D. 60°

Answer: D. 60°

Explanation: $c = -(a+b) \Rightarrow |c|^2 = |a|^2 + |b|^2 + 2|a||b|\cos\theta$. $49 = 9 + 25 + 30\cos\theta$. $\cos\theta = 1/2$.

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