Lines and Angles (NCERT Ch. 6)

• Identify adjacent, vertically opposite, and linear pairs of angles.
• Apply angle sum on a straight line (linear pair sums to 180°).
• Use vertically opposite angles are equal.
• Work with parallel lines and a transversal: corresponding, alternate interior, co-interior angles.
• Prove simple results using angle relationships.

1. Linear pair — Adjacent angles on a line sum to 180°.
2. Vertically opposite — Equal when two lines intersect.
3. Corresponding angles — Equal if lines are parallel.
4. Alternate interior — Equal if lines are parallel.
5. Co-interior (same-side interior) — Supplementary if lines parallel.
6. Converse — If corresponding angles equal, lines are parallel.
7. NCERT Ex. 6.1 — Find unknown angles in parallel line diagrams.
8. Applications — Proofs in triangles and quadrilaterals depend on this chapter.

In parallel lines cut by transversal, if one corresponding angle is 65°, find its pair.

Corresponding angles are equal when lines parallel.
Therefore the corresponding angle = 65°.
Co-interior angle on same side = 180° − 65° = 115°.

• Skipping given-to-prove structure in geometry proofs.
• Using wrong congruence criterion (SSA is not valid).
• Forgetting units in length/angle statements.

• State one NCERT result from Lines and Angles.
• Draw a neat diagram for a typical Lines and Angles problem.

Open the Practice tab for graded questions on Lines and Angles (NCERT Ch. 6).