Class 7 Maths Chapter 6 The Triangle and its Properties:

Definition: A triangle is a closed figure made up of three line segments.
A triangle consists of three line segments and three angles. In the figure above, AB, BC, CA are the three line segments and ∠A, ∠B, ∠C are the three angles.
There are three types of triangles based on sides and three based on angles.

Types of triangles based on sides

Equilateral triangle: A triangle having all the three sides of equal length is an equilateral triangle. Since all sides are equal, all angles are equal too.

Isosceles triangle: A triangle having two sides of equal length is an Isosceles triangle. The two angles opposite to the equal sides are equal.

Scalene triangle: A triangle having three sides of different lengths is called a scalene triangle. Types of triangles based on angles

Acute-angled triangle: A triangle whose all angles are acute is called an acute-angled triangle or Acute triangle. Obtuse-angled triangle: A triangle whose one angle is obtuse is an obtuse-angled triangle or Obtuse triangle. Right-angled triangle: A triangle whose one angle is a right-angle is a Right-angled triangle or Right triangle. In the figure above, the side opposite to the right angle, BC is called the hypotenuse.
For a Right triangle ABC,
BC2 = AB2 + AC2
This is called the Pythagorean Theorem.
In the triangle above, 52 = 4+ 32. Only a triangle that satisfies this condition is a right triangle.
Hence, the Pythagorean Theorem helps to find whether a triangle is Right-angled.