Given three points A (3 3,0), B (3,4) and C (0,5), what is the linear equation between the midpoint of line segment AB and point C?

Solution: The coordinates of point D in AB are ((3+3)/2, (0+4)/2) = (3,2); The coordinate of point C is (0,5); Then the CD is in a straight line

The equation can be directly obtained by two-point formula: (y-5)/(x-0) = (5-2)/(0-3) =-1,so y-5=-x, that is, x+y-5=0 is the demand.