If `veca and vecb` be two non-collinear unit vectors such that `vecaxx(vecaxxvecb)=-1/2vecb` ,then find the angle between `veca and vecb`.

Explanation

To solve the problem, we start with the given equation involving the cross product of two vectors:

**Given:**
\[ \vec{A} \times (\vec{A} \times \vec{B}) = -\frac{1}{2} \vec{B} \]

**Step 1: Use the vector triple product identity.**
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