You may have seen that $\displaystyle\frac{{1}}{{2}}\usepackage{cmbright}\Huge\$ is the negative reciprocal of $\displaystyle-{2}\usepackage{cmbright}\Huge\$ , and thought that this made the two lines perpendicular. However, these are the y-intercepts of the lines. In order for the two lines to be perpendicular, the slopes must be negative reciprocals of each other.