4. If v = -2i - 7j, then its magnitude is
||v|| = √((-2)² + (-7)²) = √53
and the unit vector in the direction of v is
[tex]\dfrac{v}{\|v\|} = \boxed{-\dfrac2{\sqrt{53}} \, \vec\imath - \dfrac7{\sqrt{53}} \, \vec\jmath}[/tex]
5. Given u = i + 2j and v = -i + 5j, we have
u • v = (i + 2j) • (-i + 5j) = -(i • i) - 2 (j • i) + 5 (i • j) + 10 (j • j)
Since i and j are orthogonal unit vectors, we have i • i = j • j = 1 and i • j = 0, so the product reduces to
u • v = -1 - 0 + 0 + 10 = 9
