In this method, you start with your thumb and first finger in an "L" shape. Then extend your middle finger so it is perpendicular to the "L". Hold your fingers in this shape, and rotate your hand so that your first finger points in the direction of →A and your middle finger points in the direction of →B. **Note: you are rotating your whole hand here, not moving your fingers at all.** (At no point should you be flipping anyone off with this Right Hand Rule.)
In this method, you start with your thumb and first finger in an "L" shape. Then extend your middle finger so it is perpendicular to the "L". Hold your fingers in this shape, and rotate your hand so that your first finger points in the direction of →A and your middle finger points in the direction of →B. **Note: you are rotating your whole hand here, not moving your fingers at all.** (At no point should you be flipping anyone off with this Right Hand Rule.)
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Then, again, your thumb will point in the direction of →C, which in this case is into the whiteboard.
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Then, again, your thumb will point in the direction of →C, which in this case is out of the whiteboard.