Persamaan suatu garis yang memiliki titik(1,2)dan titik(3,4) adalah

persamaan garisnya

(y - 2) / (4 - 2) = (x - 1) / (3 - 1)
(y - 2) / 2 = (x - 1) / 2
y - 2 = x - 1
x - y - 1 + 2 = 0
x - y + 1 = 0

persamaan garis
(y-y1)/(y2-y1) = (x-x1)/(x2-x1)
(y-2)/(4-2) = (x-1)/(3-1)
(y-2)/2 = (x-1)/2
2(y-2) = 2(x-1)
2y - 4 = 2x - 2
2y = 2x -2 + 4
2y = 2x + 2
y = (2x+2)/2
y = x + 1