x + y = w + z ...(1) | Given
∵ The sum of all the angles round a point is equal to 360°.
x + y + w + z = 3600
x + y + x + y = 3600 | Using (1)
2(x + y) = 3600
x + y = 1800
∴ AOB is a line.
| If the sum of two adjacent angles is 180°, then the non-common arms of the angles form a line.