Given that, x + y = w + z ... (1) As the sum of all angles round a point is equal to 360o $\therefore$ x + y + w + z = 360o $\therefore$ x + y + x + y = 360o $\therefore$ 2(x + y) = 360o $\therefore$ $x+y={360^0\over2}$ $\therefore$ x + y = 180o $\therefore$ AOB is a line.