If $\hat{a}, \hat{b}$ and $\hat{c}$ are unit vectors satisfying $\hat{a}-\sqrt{3}…

If $\hat{a}, \hat{b}$ and $\hat{c}$ are unit vectors satisfying $\hat{a}-\sqrt{3} \hat{b}+\hat{c}=\overrightarrow{0}$, then the angle between the vectors $\hat{a}$ and $\hat{c}$ is :
  1. $\frac{\pi}{4}$
  2. $\frac{\pi}{3}$
  3. $\frac{\pi}{6}$
  4. $\frac{\pi}{2}$

Solution

Let angle between $\hat{a}$ and $\hat{c}$ be $\theta$. Now, $\hat{a}-\sqrt{3} \hat{b}+\hat{c}=\overrightarrow{0}$ $ \begin{aligned} & \Rightarrow(\hat{a}+\hat{c})=\sqrt{3} \hat{b} \\ & \Rightarrow(\hat{a}+\hat{c}) \cdot(\hat{a}+\hat{c})=3(\hat{b} \cdot \hat{b}) \\ & \Rightarrow \hat{a} \cdot \hat{a}+\hat{a} \cdot \hat{c}+\hat{c} \cdot \hat{a}+\hat{c} \cdot \hat{c}=3 \times 1 \\ & \Rightarrow 1+2 \cos \theta+1=3 \\ & \Rightarrow \cos \theta=\frac{1}{2} \Rightarrow \theta=\frac{\pi}{3} \end{aligned} $

Asked in: JEE Main 2013 (22 Apr Online)

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