From Figure 1.1.1 and the recipe $∥\mathbf{V}∥\=\sqrt{a^2\+b^2}$, the magnitude (or length) of the given position vector is $\sqrt{1^2\+2^2}\=\sqrt{5}$.

With Student MultivariateCalculus, (or LinearAlgebra, or VectorCalculus) loaded, the norm defaults to the Euclidean norm. This norm in the Student MultivariateCalculus package can be accessed either from the Context Panel, or directly, with Typeset math.

Here are two top-level solutions (no packages installed).

The subscript on the norm forces Maple to compute the 2-norm, that is, the Euclidean norm. At top level, with no relevant package loaded, Maple's norm (computed via Typeset math notation) is the largest absolute value of the components, the default for the LinearAlgebra package.

The Norm command exists only in certain packages. Hence, at top level, without one of those packages being loaded, the Norm command returns unevaluated. Within the LinearAlgebra package, the Norm command defaults to the infinity norm, the largest absolute value of the components. To obtain the Euclidean norm in the LinearAlgebra package, an additional parameter (either "2" or "Euclidean") must be added. In the Student MultivariateCalculus, LinearAlgebra and VectorCalculus packages, Norm  defaults to the Euclidean norm.