Show that the line joining the points A(4, 8) and B(5, 5) is parallel to the line joining the points C(2,4) and D(1,7).

Proof
Given co-ordinates of A = (4,8)
Co-ordinates of B = (5,5)
Slope of line AB = (y₂-y₁)/ (x₂-x₁)
= (5-8)/(5-4)
= -3/1 = -3
Given co-ordinates of C = (2,4)
Co-ordinates of D = (1,7)
Slope of line CD = (y₂-y₁)/ (x₂-x₁)
= (7-4)/(1-2)
= 3/-1 = -3
Slope of line AB = Slope of line CD
Line AB is parallel to line CD.
Hence proved.