## Geometry: Common Core (15th Edition)

We know that if the slope of two lines are opposite reciprocals, they are perpendicular. To check whether the slopes are opposite reciprocals, we put both equations in slope intercept form. The first equation is already in slope intercept form. Rearranging the other equation gives: $y = x+20$ From this, we see that the two lines are perpendicular as the slopes are opposite reciprocals.