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A triangle is a geometric shape that has three sides and three angles. When constructing a triangle the length of the sides can be chosen freely as long as none of the sides are longer than the sum of the other two. The sum of all angles in a triangle is always 180°.

The area of a triangle can most easily be calculated when the *base* and *height* is known.

Area =

base×height

2

2

The base is one of the triangle's three sides. The height is the line that you get if you draw a perpendicular line from the base to the opposite corner. Note that the tool on this page uses b as base.

Triangles are often categorised based on their appearance.

Acute | All angles are less than 90°. |

Right-angled | One angle is exactly 90°. |

Obtuse | One angle is bigger than 90°. |

Isosceles | Two sides have the same length. |

Equilateral | All sides have the same length. |

A triangle is either acute, right-angled, or obtuse. It is impossible for a triangle to be more than one of these at the same time. An equilateral triangle is always acute.

Knowing the type of triangle can sometimes be helpful when trying to figure out other information about a triangle. When using this tool it is therefore important that you specify if the triangle is isosceles (by using the check box) or right-angled (by specifying one of the angles as 90°).