Class 7 Maths Part 1 Chapter 7, A Tale of Three Intersecting Lines, explores the geometry and compass-and-ruler construction of triangles—the triangle inequality theorem, SSS/SAS/ASA constructions, the angle sum property ($180^\circ$), exterior angles, altitudes and classification of triangles.
What Class 7 Mathematics Chapter 7 Covers
7.1–7.2 Equilateral Triangles, SSS Construction & Triangle Inequality constructs equilateral and scalene triangles with compass and straightedge when three side lengths are given, and proves the Triangle Inequality: a triangle exists if and only if each side length is strictly less than the sum of the other two sides.
7.3 SAS/ASA Constructions, Angle Sum & Exterior Angle Properties constructs triangles given two sides and the included angle (SAS) or two angles and the included side (ASA), and proves using parallel lines that the three interior angles of any triangle add up to $180^\circ$ and an exterior angle equals the sum of the two opposite interior angles.
7.4–7.5 Altitudes & Types of Triangles constructs the perpendicular altitudes of acute, right and obtuse triangles using paper folding and a set square, and classifies triangles by sides (equilateral, isosceles, scalene) and by angles (acute, right, obtuse).
Exercises in Chapter 7
- Figure it Out exercise sets and Summary with solved walkthroughs.
Read Class 7 Mathematics Chapter 7 Online
More Chapters
- Previous: Chapter 6: Number Play
- Next: Chapter 8: Working with Fractions
- Hindi medium: Class 7 Maths Part 1 Chapter 7 PDF in Hindi (Ganita Prakash)
- All Mathematics chapters and the other Class 7 books: Class 7 NCERT books 2026-27
FAQ
When is it possible to construct a triangle with three given side lengths?
A triangle can be constructed if and only if the sum of the lengths of any two sides is strictly greater than the length of the third side (the Triangle Inequality).
What is the Angle Sum Property and the Exterior Angle Property of a triangle?
The sum of the three interior angles of any triangle is always $180^\circ$, and the measure of any exterior angle of a triangle is equal to the sum of its two opposite interior angles.