Concept Slope intercept form of a straight line ⇔y=mx+c Where m is the slope and c is the y-intercept When two lines are perpendicular, the product of their slope is −1. If m is the slope of a line, then the slope of a line perpendicular to it is −1∕m. Calculation: Given equations are, y+px=1 and y−qx=2 which can be written as, y=−px+1 and y=qx+2 So, m1 (slope of 1 st line )=−p and m2 (slope of 2 nd line )=q As they are perpendicular m1×m2=−1 ⇒−pq=−1 ⇒pq−1=0