**Basic Concepts of Geometry Workbook-2**

**A triangle is a three sided closed figure**

A triangle whose all the sides are unequal is called a scalene triangle. All the three angles will also be different in this case.

AB ≠ BC ≠ AC

A triangle, two of whose sides are equal in length is called an isosceles triangle. In this case two angles will be of equal measure and one will be different.

AB = AC ≠ BC

∠B = ∠ C □ ≠ ∠A

A triangle, all of whose sides are equal is called an equilateral triangle. All the three angles will be equal in this case and each of them will be of 60 degree measure.

AB = BC = AC

∠A = ∠B = ∠C = 60°

A triangle, each of whose angles is less than 90°, is called an acute angled triangle.

**Acute Angle triangle**

A triangle with one of its angle 90° is called a right angled triangle.

A triangle with one of its angle greater than 90° is known as an obtuse angled triangle.

**Obtuse Angle triangle**

If a side of a triangle is produced, the exterior angle so formed is equal to the sum of two interior opposite angles.

∠x = ∠A + ∠B

∠A + ∠B +∠C = 180° ....(1)

∠C + ∠x = 180° (linear pair) ....(2)

From (1) & (2)

∠x = ∠A + ∠B

**Important : The angle opposite to biggest side is the biggest and the angle opposite to smallest side is the smallest.**