Changed article from a stub to basic information (#24480)

* Update index.mb

Article was a stub, added relevant information based on the Circle page and classifications of triangles

* Added formatting

guide/english/mathematics/triangle/index.md

@@ -3,13 +3,28 @@ title: Triangle
 ---
 ## Triangle
 
-This is a stub. <a href='https://github.com/freecodecamp/guides/tree/master/src/pages/mathematics/triangle/index.md' target='_blank' rel='nofollow'>Help our community expand it</a>.
+A triangle is a polygon which has three sides and vertices, whose internal angles add up to 180 degrees.
 
-<a href='https://github.com/freecodecamp/guides/blob/master/README.md' target='_blank' rel='nofollow'>This quick style guide will help ensure your pull request gets accepted</a>.
+Any side of a triangle can be considered a base (B).
 
-<!-- The article goes here, in GitHub-flavored Markdown. Feel free to add YouTube videos, images, and CodePen/JSBin embeds  -->
+The distance from the base to the vertice that does not touch it is the height (H).
+
+### Classifications
+
+- A right triangle is a triangle that contains one angle equal to 90 degrees.
+
+- An acute triangle is a triangle that consists entirely of angles less than 90 degrees.
+
+- An obtuse triangle is a triangle that contains one angle more than 90 degrees.
+
+- An equilateral triangle's 3 sides and angles are always equal in length and degree (and therefore the angles are equal to 60 degrees).
+
+- An isosceles triangle has 2 sides and angles that are equal in length and degree.
+
+- A scalene triangle has no sides or angles that are equal in length or degree.
+
+![triangle](https://upload.wikimedia.org/wikipedia/commons/e/eb/Regular_polygon_3_annotated.svg "triangle")
 
 #### More Information:
 <!-- Please add any articles you think might be helpful to read before writing the article -->
-
-
+- [Wikipedia](https://en.wikipedia.org/wiki/Triangle)