The Triangle
Definition of a Triangle
Triangle is a closed figure bounded by three straight lines called sides. It can also be defined as polygon of three sides.
![triangle.jpg](/sites/default/files/users/Mathalino/plane-geometry/triangle.jpg)
Area of triangle
The area of the triangle is given by the following formulas:
$A = \frac{1}{2}bh$
![001-area-of-triangle.gif](/sites/default/files/users/Mathalino/plane-geometry/001-area-of-triangle.gif)
Given two sides and included angle
$A = \frac{1}{2}ab \sin \theta$
Given three sides (see the derivation of Hero's formula)
$A = \sqrt{s(s - a)(s - b)(s - c)}$
where, $s = \frac{1}{2}(a + b + c)$ called the semi-perimeter.
Given one side and three angles (say angles A, B, and C, and side b are given)
$Area = \dfrac{b^2\sin A~\sin C}{2\sin B}$
See also the links below:
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