Chapter 13 - Vector Geometry - 13.4 The Cross Product - Exercises - Page 677: 29

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If $ v $ and $ w $ lie in the $ zy $-plane, then according to the right-hand rule, $ v\times w $ will be perpendicular to the $ zy $-plane; that is, it will point in the direction of $ x $ and hence it will be a multiple of $ i $ ($i$ is the unit vector pointing along the $x$ axis).