Solve this following

Question:

Let $\hat{a}$ and $\hat{b}$ be two unit vectors. If the vectors $\vec{c}=\hat{a}+2 \hat{b}$ and $\vec{d}=5 \hat{a}-4 \hat{b}$ are perpendicular to each other, then the angle between $\hat{a}$ and $\hat{b}$ is :

  1. $\frac{\pi}{4}$

  2. $\frac{\pi}{6}$

  3. $\frac{\pi}{2}$

  4. $\frac{\pi}{3}$


Correct Option: , 4

Solution:

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