Chapter 11 - Section 11.3 - The Dot Product - Exercises - Page 617: 33

$x+2y=4$

The result of exercise 31 tells us that ${\bf v}=\langle a,\ \ b \rangle$ is perpendicular to the lines $ax+by=c$. Here, ${\bf v}=\langle 1,\ \ 2 \rangle$, and the lines are $x+2y=c.$ Since P(2,1) is on the line, its coordinates satisfy the line equation $(2)+2(1)=c$ $c=4$ So the line equation is: $\quad x+2y=4$ To graph, we use: intercepts: $(4,0)$ and $(0.2)$