Introduction
Triangles are the simplest form of Polygon. The word Polygon means a plane figure which has many line segments connected end to end. A single or double line segments together can never form a polygon. The line joining any three dots can form a triangle or any three line segment connected to each other end to end also forms a triangle. A triangle is a threesided closed 2D shape having three vertices as well as three angles.
Terms related to Triangles:
Median: A line segment joining a vertex to the midpoint of the opposite side of a triangle is called a median of a triangle. In figure ABC, D is the midpoint of AB. Thus AD forms the median of the triangle ABC. Similarly, a median can be drawn from the midpoint of BC as well as CA. In other words, a triangle can have three medians.
Centroid: The point of intersection of the three medians of a triangle is called the centroid. Here, the midpoint of the medians AD, BE and FC is the centroid of the triangle ABC.
Altitude: The length of the perpendicular from a vertex to the opposite side of a triangle is called its altitude, and the side on which the perpendicular is drawn is called its base.
In the triangleABC, the perpendicular drawn to BC, that is AL is the altitude. The side BC is called the base of the triangle.
Orthocentre: The point of intersection (or concurrence) of the three altitudes of a triangle is called its orthocentre.The meeting point (H) of the altitudes AL, CN and BM of the triangle is called the orthocentre.
Incentre and Incircle: The point of intersection of internal bisectors of the angle of a triangle is called incentre. Here, the point I which is the meeting point of the bisectors of the angles A, B and C is called Incentre. The incentre of a circle is also the centre of the circle which touches all the sides of the triangle.
Circumcentre and Circumcircle: The point of intersection of the perpendicular bisectors of the sides of a triangle ABC is called its circumcentre. In the figure, the perpendicular bisectors of sides AB, BC and CA of the triangle ABC intersects at point O. The point O is called the circumcentre of the triangle.Circumcircle is the circle drawn keeping the circumcentre of the triangle as the centre such that the circle passes through all the vertices of the triangle.
Properties of a Triangle:

Angle sum property of Triangle
Angle Sum Property of a Triangle states that the sum of all the three angles of a triangle is equal to 180 degrees.

Exterior angle Property
Each side of a triangle can be extended both ways. The three sides of a triangle give rise to six extended sides, each side making to extended sides. An exterior angle is an angle between one side and one extended side of a triangle. There are three extended sides in a triangle and each extended side results in one exterior angle, therefore, a triangle has six exterior angles. In the diagram given below, angle 1, 2, 3, 4, 5 and 6 are exterior angles of the triangle.
Note: The angles between two extended sides are not Exterior angles.
Exterior Angle Property states that:

The sum of any two interior angles of a triangle is equal to the opposite exterior angle.

The sum of the equivalent exterior angle of a triangle is always equal 360 degrees.
2) Properties of Similar Triangles
Similar triangles have the same basic shapes. Triangles having the same angles but different sizes are called Similar Triangles as the ratio of the sides remains the same. The rotated triangles or the mirror image triangles are also called Similar triangles as the angles and sizes are same.

The ratio of the corresponding sides of two similar triangles is same.

The corresponding angles of two similar triangles are congruent.
Similar Triangles can be identified in three ways:

Angle Angle Angle: Two triangles are said to be similar triangles if all the three sides of one triangle are equal to the corresponding angles of the other triangle.

Side Side Side: Two Triangles are said to be similar if the sides of one triangle are equal or in the same ratio to the corresponding sides of another triangle.

Side Angle Side: Two triangles are said to be similar if the two sides of one triangle are equal or in a ratio to the corresponding sides of the other triangle provided the included angles are also congruent.
3) Median Property of a Triangle
1) It bisects the angle at the vertex of an isosceles and equilateral triangle whose adjacent sides are equal.
2) It also bisects the angle of the vertex of an isosceles and equilateral triangle.
3) There are only three medians of a triangle.
4) The point of intersection of the medians is called the centroid.
5) It divides the area of a triangle in two halves.
6) The length of each median is divided in the ratio 2:1 by the centroid.
7) The centroid divides the triangle into 6 smaller triangles of equal area.
8) All the medians of equilateral triangles are equal.
9) The medians drawn from vertices of an isosceles triangle with equal angles are equal in length.
10) The length of all the medians of a scalene triangle is different.
11) Apollonius Theorem can be used to find the length of the medians.

Properties of Circumcenter of a Triangle

The centre of the circumcircle is the circumcentre of the triangle.

The vertices are at equal distance from the circumcenter of a triangle.

For the acuteangled triangle, the circumcenter is always inside the triangle.

For the obtuseangled triangle, the circumcentre is outside the triangle.

For the rightangled triangle, the Circumcenter is at the midpoint of then hypotenuse.

Properties of Perpendicular Bisector

It is the bisector of a line segment.

It intersects the line segment at 90 degrees

It passes through the line segment’s midpoint.

The points on the perpendicular bisector of a segment are always equidistant from the endpoints of the segment.

Similarly, if a point is equidistant from the endpoints of a segment, then it lies on the perpendicular bisector of the segment.

It shows that locus of points equidistant from the endpoints which meet at the midpoint of the segment at a right angle exists. Points E, F, G, and H are equidistant from A and B. Points E, F, G, and H together form the perpendicular bisector of segment AB.

Properties of Circumcentre of a triangle
1) It is the point of intersection of the perpendicular bisectors of the sides of a triangle
2) It is equidistant from the vertices of the triangle.

Properties of Angle Bisector of a triangle

A point on the angle bisector of the triangle is equidistant from its sides.

There is always a common point at which the angle bisectors of a triangle meet.

It proves the congruency between two angles.
8) Properties of Incentre of a triangle
1)It is the intersection point of the angle bisector of a triangle
2) It is equidistant from the sides of the triangle.
9) Properties of centroid of a triangle
1) It is the intersection of three medians of a triangle.
2) It is a point of congruency of a triangle.
3) It is always on the inside of a triangle.
4) It acts as the centre of gravity of a triangle.
5) It divides each median in the ratio 2:1.
Classification of Triangles on the basis of sides and their properties:

Scalene Triangle
A triangle in which all the sides are of different length is called Scalene Triangle. In the adjoining figure, all the sides of the triangle are of different length (i.e, 13, 9 and 14 cm).
Properties:

A scalene triangle has all the sides different.

It has unequal angles.

Its longest side is right opposite to its biggest angle.

It cannot be bisected into two equal halves.

It has no line of symmetry.

A scalene triangle can be an acute scalene triangle, an obtuse scalene triangle or right scalene triangle.

The area of a scalene triangle can be calculated by using Heron’s formula if all the sides are given.

When a scalene triangle is inscribed in a circle, each angle is half the angle subtended by the opposite side.

The centre of the circumscribing circle lies inside the triangle if all the three angles are acute.

Isosceles Triangle
A triangle having two sides of equal length is called Isosceles triangle. In an isosceles triangle, the angles opposite to the equal sides are equal. In the triangle given below, two sides are of 5inches and one side is of 3 inches. Thus, it is an Isosceles Triangle.
Properties:

Two sides are congruent to each other.

The unequal side of an isosceles triangle is called base.

The two angles opposite to the equal sides are congruent to each other. Thus it has two two congruent base angles.

Apex angle is the angle which is not congruent to the two base angles which are congruent.

The height drawn from the apex of an isosceles triangle divides the base into two equal parts and also divides the apex angle into two equal angles.

Area of Isosceles triangle = ½ × base × height

The perimeter of an Isosceles triangle = sum of all the three sides

The third unequal angle of an isosceles can be acute or obtuse.

The circumcenter of an isosceles triangle lies inside the triangle if all the three angles of the three triangles are acute.

The sides of the triangle are the chords of the circumcircle.

If one of the angles is 90 degrees, then the circumcenter lies outside the triangle.

The centroid is the intersection of the medians of the Isosceles triangle.

The median drawn from Apex divides the triangle at right angles.

The perpendicular bisectors of an isosceles triangle intersect at its circumcenter.

The angle bisectors of an isosceles triangle intersect at the incenter.

The circle drawn with the incenter touches the three sides of the triangle internally.

Each median divides the isosceles triangle into two equal triangles having the same area.

The area of the triangle can be estimated:

If the measure of one angle and one side are given

If three sides of the triangle are given.

If two sides of an isosceles triangle and their included angles are given.

Joining the midpoint of three sides divides the triangle into 4 smaller triangles of the same area.

When a circle with the diameter equal to the base is drawn:

For an obtuseangled isosceles triangle, the apex lies inside the circle.

For a rightangled isosceles triangle, the apex lies on the circumference.

For an acuteangled isosceles triangle, the apex lies outside the triangle.

When the midpoint apex is taken as a radius and a circle is drawn with the midpoint of the base as the centre

For an acuteangled isosceles, the base vertices lie inside the circle.

For a rightangled isosceles the base vertices lie on the circumference

For an obtuseangled isosceles triangle, the base vertices lie outside the circle.

Equilateral Triangle
A triangle whose all the three sides are of equal length is called an equilateral triangle. The measure of each angle of a triangle is 60 degrees. The triangle given below is an equilateral triangle because all the sides are equal (i.e, 4cm) with angle 60 degrees each.
Properties:

All the sides of an equilateral triangle are always the same.

All the angles of the equilateral triangle are the same.

The median, altitude, angle bisector, perpendicular bisector all coincide at one line.

The median, altitude, perpendicular bisector and angle bisector forms the line of symmetry of an equilateral triangle.

The length of all the medians, altitude, perpendicular bisector and angle bisector are the same.

The area of an equilateral triangle is 23 /4 s2. Here, s is the sides of an equilateral triangle.

The orthocenter, circumcenter, incenter and centroid all lie at the same point.

Each altitude is a median of the equilateral triangle.

The centroid is the meeting point of the angle bisectors, medians as well as perpendicular bisectors of a triangle.

The incenter and the circumcenter of an equilateral triangle are the same.

The area of an equilateral triangle can be estimated:

If the measure of one angle and one side are given

If three sides of the triangle are given.

If two sides of the triangle and their included angles are given.
Classification of Triangles on the basis of angles and their properties:

Acute Triangle
A triangle in which every angle measures more than zero degrees but less than 90 degrees is called acuteangled Triangle. All the interior angles of the triangle given below are 30, 70 and 80 degrees i.e, all are less than 90 degrees.
Properties:
1. It has all three angles as acute.
2. Its perpendicular bisectors intersect at the circumcenter and median intersect at the centroid.
3. The circumcenter of an acute triangle lies inside the triangle.
4. The angle bisectors meet at the incenter of the circle. A circle can be drawn with the incenter of the triangle as the centre of the circle to touch the three sides of the triangle internally.
5. Joining the midpoints of the three sides of the triangle results in 3 parallelograms having the same area and 4 triangles of the same area.

Obtuse Triangle
A triangle in which one of the angles measures more than 90 degrees but less than 180 degrees is called an obtuseangled triangle. In the given triangle, one angle that is 120 degrees which is greater than 90 degrees whereas the other two angles are less than 90 degrees.
Properties:
1. It has two acute angles and one obtuse angle.
2. Its perpendicular bisectors intersect at the circumcenter.
3. The medians intersect at the centroid.
4. Its circumcenter always lies outside the triangle.
5. The angle bisectors meet at the incenter of the triangle. A circle drawn with the incentre as its centre touches the three sides of the triangle internally.
6. Medians divide the triangle into two smaller triangles having the same area.
7. Joining the midpoints of the three sides of the triangle results in 3 parallelograms of the same area and 4 triangles of the same area.

Right Triangle
A triangle in which one of the measures of the angles exactly 90 degrees is called a right triangle. In the triangle given below, one of the three angles of the triangle is 90 degrees whereas the other two angles are less than 90 degrees.
Properties:
1. Out of the three angles of a rightangled triangle, one angle is greater than 90 degrees and the other two are acute angles.
2. The longest side of the right triangle is called hypotenuse and the angle opposite to hypotenuse is 90 degrees.
3. The area of a right triangle is half the product of the base and height.
4. The radius of the circumcircle is always half the hypotenuse and the centre of the circumcircle is always the midpoint of the hypotenuse.
6. A line perpendicular to hypotenuse from the right angle results in three similar triangles.