Triangle inequality theorem

As we all are very well aware that geometry provides lots of shapes and figures to solve out the problems that are related to mathematics. In mathematics, triangle is a shape that provides the various concepts to solve the problem. Triangle inequality is one of them, which states that the sum of lengths of any two sides of the triangle must be greater than or equal to the length of third side. Euclidean geometry says that triangle inequality is a theorem about the distance. In simple definition of the triangle inequality theorem we can say that the total of any two sides of triangle must be greater than the third side of the triangle. If in case the above theorem is not applied at the time of triangle creation then that figure is not considered as a Triangle. This theorem helps the student of Grade IX to understand the concept.
Suppose a triangle has three sides which named as A, B and C. Now we check that the given triangle is a complete triangle or not by applying the inequality theorem. The Triangle Inequality theorem says that:
B + C > A,
A + B > C,
C + A > B,
If in the given triangle, available values satisfy the above combination or pairs of inequality then the triangle can be considered as complete triangle. In study of Triangle of geometry, we generally study three types of triangle which are equilateral triangle, isosceles triangle and right angle. In all types of triangle we can easily see the operation of triangle inequality theorem. (know more about icse syllabus 2013 , here)
Example: Can a triangle have the side lengths of side 1 = 4cm, side 2 = 8 cm and side 3 = 2cm?
Solution: Now we need to apply the triangle inequality theorem. By applying the Triangle inequality theorem we create three combinations that are given below:
Side 1 + side 3 > side 2,
  4 + 2 > 8,
The first equation get false then we can say that the above given sides are not able to form the triangle.
In the next session we will discuss about Basic constructions. 

Angles of triangles and polygons

Hello students here we are going to discuss the angles of triangles and angles of polygons. But before discussing it you should get familiar with the term 'angle'. If two rays OP and OQ have a common end point ‘O’ then it form angle POQ, written as ∠POQ.

Angle may be interior, if any point in its (∠ POQ) plane lies on the same side of OP as Q and also on the same side of OQ as P and exterior of an angle has points in its plane which do not lie on the angle or in its interior.

Angle is of following types such as: right, acute, obtuse, straight, reflex and complete angles.

Let us take a look at Angles of triangles: Suppose we have a triangle ∠ABC, the interior of this triangle is the set of all points in its plane, which lies on the same side of OP as ‘Q’ and also on the same side of OQ as P or in other words if any angle is inside the boundary of the triangle then it said to be interior angle of the triangle and if any angle is outside the boundary of the triangle then it said to be exterior angle

Note: - The interior angles of a triangle can add up to the 180°. (know more about icse class 10 books, here)

Angles of polygons: Polygons are the shapes that have more than two angles or have straight sides. Example triangle, rectangle, and rhombus, etc. Interior angles are those which lie inside the boundary and vice versa.

Note: The exterior angles of a polygon can add up to the 360°.

 Above information will be useful for the Grade IX students.

In the next session we are going to discuss Error and magnitude.

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