Let the angles of the quadrilateral be a°, (a + d)°, (a + 2d)° and (a + 3d)°.
Since sum of all angles of a quadrilateral is 360°, we have.
a° + (a + d)° + (a + 2 d)° + (a + 3 d)° = 360° ⇒ 4a + 6d = 360°
⇒ 2a + 3d = 180° …(1)
Now, greatest angle is 120°
So, a + 3d = 120° …(2)
Solving (1) and (2), we have
a = 60°, d = 20°
Hence, the angles are 60°, 80°, 100°, 120°.
