[[!redirects triangles]] # Triangle A **triangle** is a [[polygon]] with three sides. ## Types of Triangle * [[Equilateral Triangle]] * [[Isosceles Triangle]] * [[Right-angled Triangle]] * [[Scalene Triangle]] ## Angles in a Triangle The [[interior angles]] in a triangle add up to $180^\circ$. ## Area of a Triangle There are several ways to calculate the area of a triangle. 1. Choose one side as the "base" and measure the "height" of the third vertex above this base. Then the area is $\frac{1}{2} \times \text{base} \times \text{height}$. 2. Let $a$ and $b$ be the lengths of two of the sides and $C$ the angle between them. Then the area is $\frac{1}{2} a b \sin(C)$. 3. Let $s$ be the _semi-perimeter_ (that is, half the [[perimeter]]). And let $a$, $b$, $c$ be the side lengths. Then the area is $\sqrt{s(s - a)(s - b)(s - c)}$. [[!redirects triangles]] [[!redirects angles in a triangle]] [[!redirects area of a triangle]] [[!redirects triangle area]] [[!redirects triangle areas]] category: angle facts category: shapes