11a94d…: “Despite the heavy arc”

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Let $\vec {u}$ and $\vec {v}$ be vectors in $\mathbb {R}^3$ . Suppose $|\vec {u}|=3$ , $|\vec {v}|=7$ and that the angle between $\vec {u}$ and $\vec {v}$ is $\theta =\frac {\pi }{2}$ . Compute $|\vec {u}\cross \vec {v}|$ . $|\vec {u}\cross \vec {v}|=\answer {21}$