A triangle has vertices P (6, 7), Q(2 -1) and R(-2, 1). How do I use the slopes of the sides to show that PQR is right angled.
Two lines are perpendicular if their slopes are negative reciprocals.
The slope of the line between (-2, 1) and (2, -1) is:
(1 - (-1)) / (-2 - 2) = 2 / -4 = -1/2
The slope of one of the other lines is 2. (your job to figure out which one)
Those two lines are negative reciprocals - so they are perpendicular.
So the triangle is a right triangle.