Mathematics

Problem

17. Let

\vec{a}

and

\vec{b}

be two unit vectors and

\theta

is the angle between them. Then

\vec{a}+\vec{b}

is a unit vector if (A)

\theta=\frac{\pi}{4}

(B)

\theta=\frac{\pi}{3}

(C)

\theta=\frac{\pi}{2}

(D)

\theta=\frac{2 \pi}{3}

(A)

\theta=\frac{\pi}{4}

(B)

\theta=\frac{\pi}{3}

(C)

\theta=\frac{\pi}{2}

(D)

\theta=\frac{2 \pi}{3}