The Total solution for NCERT class 6-12
In the figure,
l1 ||l2 and l3 || l4 and m is theangle bisector∴ ∠2 = ∠3 = y∵ l1 ||l2∠1 = x(Corresponding angles)∵ l3 ||l4∴ ∠1 + (∠2 + ∠3) =180° (Co-interior angles)⇒ x +2y= 180°⇒ 2y=180°-x