If ` vec a\ a n d\ vec b` be two unit vectors and `theta` is the angle between them. Then ` vec a+ vec b\ ` is an unit vector, if `theta=` `pi/2` b. `(2pi)/3` c. `pi/4` d. `pi/3`

Explanation

To determine the angle \( \theta \) for which the vector sum \( \vec{a} + \vec{b} \) is a unit vector, we can follow these steps:

### Step 1: Understand the properties of unit vectors
Both \( \vec{a} \) and \( \vec{b} \) are unit vectors, which means:
\[ |\vec{a}| = 1 \quad \text{and} \quad |\vec{b}| = 1 \]

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