Triangle

• Line: It is the union of a sequence of points, which are located in a linear way, that is, there are no curves between them.
• Segment: It is a portion of a line, which is joined between two points.
• Height: The height of a triangle is the measure or length from a vertex to the highest point of the triangle, it is usually denoted with the letter h.
• Median: the median is the segment that extends from a given vertex to the midpoint of the opposite side to that vertex.
• Bisector: the bisector is a ray that extends from a given vertex, dividing it into two equal angles.
• Perpendicular bisector: the perpendicular bisector is the line that exactly halves its sides and can be drawn perpendicular to those sides.
• Middle segment: In the case of triangles, the middle segment is that line that we can draw by locating the midpoint of two sides, this line will measure half of the third side.
• Opposite side: an opposite side is one that is located in front of a given vertex.

The triangle is a figure composed of $3$ sides and the sum of all its angles always equals $180$ degrees.
There are several types of triangles:

Equilateral triangle - All sides (or edges) are equal, all angles are equal, and all heights are also the median and the bisector.
Isosceles triangle - It has two equal sides, two equal base angles, and the median is also the height and the bisector.
Right triangle - It has an angle of $90$ degrees formed by two legs. The side opposite the right angle is called the hypotenuse.
Scalene triangle - All sides of the triangle are different.

Click here for a more in-depth explanation about the types of triangles.

In any triangle, regardless of the type of triangle it is, the sum of all its angles equals $180 º$.
In the equilateral triangle -> each angle is $60 º$ degrees.
In the isosceles triangle -> the two base angles are equal and the third completes the $180 º$.
In the right triangle -> only one angle is $90 º$ and the other two complete the $180 º$.

Another note:
In the special triangle of 90 º , 45 º , 45 º -> only one angle is $90 º$ and the other two are $45 º$ each, this creates a triangle that is both isosceles and right at the same time.

Exercise:
Given the following angles:
angle $A=80 º$
angle $B=50 º$
angle $C=50 º$

Click here for a more in-depth explanation about the angles of triangles.

Here we will present the general formula for calculating the area of triangles:

This formula is used to calculate the area of isosceles, equilateral, and scalene triangles.
Right triangle

length of the first leg $\times$ length of the second leg
\frac{length~of~the~first~leg~\times ~length~of~the~second~leg

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The perimeter of the triangle is equal to the sum of the lengths of all sides.

In an equilateral triangle – all sides are equal, therefore, the perimeter of the triangle will be $3\cdot side$
In an isosceles triangle - there are two equal sides and it is convenient to remember this when we want to deduce the perimeter

Click here for a more in-depth explanation about the perimeter of the triangle.

Triangles are considered congruent if all their angles and all their sides are equal respectively.
To prove that two triangles are congruent you must demonstrate one of the following congruence theorems:

ASA – angle, side, angle
If both triangles have 2 equal angles and the length of the side between them is also equal, the triangles are congruent.

SAS – side, angle, side
If both triangles have 2 equal sides and the adjacent angle is also equal, the triangles are congruent.

SSS - Side, side, side
If the lengths of all 3 sides are equal respectively in both triangles, the triangles are congruent.

SSA - Side, side, angle
If the 2 sides are equal in both triangles and so is the angle opposite the larger side, the triangles are congruent.

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Similar triangles do not need to have identical areas as is the case with congruent triangles, it is enough that they have the same proportions.
To prove that two triangles are similar you must demonstrate one of the following similarity theorems:

AA – Angle, Angle
If two angles of one triangle are equal to two angles of the other, the triangles are similar.

SSS - Side, Side, Side
If in one triangle the three sides are proportional to the three sides of the other, the triangles are similar.

Click here for a more in-depth explanation on the similarity of triangles.