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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`

A

`theta = (pi)/(4)`

B

`theta =(pi)/(3)`

C

`theta=(pi)/(2)`

D

`theta=(2pi)/(3)`

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The correct Answer is:
D
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