

Let the points are A(x, y), B(a, 0) and C(0, b).
If the points A, B, C are collinear, then the area of the triangle will be equal to zero
=> (1/2)*{x(0 - b) + a(b - y) + 0(y - 0)} = 0
=> -bx + ab - ay = 0
=> bx + ay = ab
=> bx/ab + ay/ab = 1
=> x/a + y/b = 1
