Chapter 9 - Polar Coordinates; Vectors - 9.6 Vectors in Space - 9.6 Assess Your Understanding - Page 625: 5

False.

By definition the dot product of $2$ vectors $ai+bj$ and $ci+dj$ is: $(ai+bj)(ci+dj)=(ac)i+(bd)j.$ Hence, if for example $a,b$ are negative and $c,d$ are positive then the dot product is negative since both $ac$ and $bd$ are negative. . Thus, the given statement is false.