Class 7 · Chapter 6

The Triangle and its Properties

Types of triangles; angle sum property; exterior angle; Pythagoras' theorem.

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Tip: open the NCERT PDF for the original chapter text alongside the notes here.

Detailed theory

By sides: scalene (all different), isosceles (two equal), equilateral (all equal). By angles: acute, right, obtuse.

Key concepts & formulas

Sum of interior angles of a triangle

= 180^\circ

Exterior angle

=

sum of two opposite interior angles.

In a right triangle:

\text{hyp}^2 = \text{base}^2 + \text{height}^2

Two triangles are congruent if their corresponding sides and angles are equal.

In a triangle, the sum of any two sides is greater than the third.