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Proof (c) If , then , so
Proof (d)
The result now follows on taking square roots of both sides.
Part (c) of this theorem states that multiplying a vector by a scalar k multiplies the length of that vector by a factor of (Figure
4.1.2a). Part (d) of this theorem is known as the triangle inequality because it generalizes the familiar result from Euclidean
geometry that states that the sum of the lengths of any two sides of a triangle is at least as large as the length of the third side
(Figure 4.1.2b).
Figure 4.1.2
The results in the next theorem are immediate consequences of those in Theorem 4.1.4, as applied to the distance function
on . They generalize the familiar results for and .
THEOREM 4.1.5
