Narrator: These two lines are perpendicular, which just means they are on either side of a right angle. The lines facing each other are parallel. This means they’re always the same distance apart.

Using these two rules it's possible to prove that angles in a triangle add up to 180. Back to the bridge. To start, draw two parallel lines.

Class 9 Maths Chapter 6 Lines and Angles is available here for download in PDF format. We have provided here the latest edition of the chapter which is published by the NCERT on its official website.