We were given a graded assigment and one of the question asks. Prove that all curves in the family y1=-.5x^2 + k (k any constant) are perpendicular to all curves in the family y2=lnx+ c (c any constant) at their points of intersection. I found the derivatives of y1 and y2 and they are reciprocal to each other. But it is asking for points of intersection of the curves. Can curves be perpendicular to each other. The graded assigment is due tomorrow if possible if you give an explanation right about now it would be really great.