Let the quadrilateral be ABCD; It is given that angle A+C(or any two opp. angle) = 180^ .Then sum of angle B+D will also be equal to 180^(i.e. supplymentry). Because angle A+B+C+D = 360^.
Sum of interior angles of a polygon is given by - [ (n-2)180^ ] , where n is no. of sides of polygon.