Prove that a vector perpendicular to the line $Ax+By+C=0$ in the $xy$-plane is $$
So intuitively this is clear, since we can always find a vector normal to a line by looking at the coefficients of $x,y,z$ So in this case $$ or $$ in 2d space is clearly a normal vector of the line.
I tried using this approach but got stuck : take two points on the line, find a vector parallel to the line, take the dot product with $$ and show this is $= 0$
So
Let $P_1 = Ax_1 + By_1 = -C$ be a point on the line and
Let $P_2 = Ax_2 + By_2 = -C$ be another point on the line.
(let this denote the vector) Then $P_1P_2 =
So