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A plane triangle is an object having no thickness and 3 straight sides. (They are flat, or two-dimensional. Triangles in spaces with more dimensions, for example a triangle on the surface of a globe, are not described here.) Every triangle has 3 sides, 3 vertices (the points where the sides meet), and 3 angles.
If the sizes of three of the six parts (3 angles and 3 sides) of a triangle are known, and at least one of the known parts is a side, the sizes of the other sides and angles can be calculated. Mathematicians call this, “solving the triangle.”
In what follows, the sides are named with lowercase letters, and the angle opposite a side is named by the same letter, but in uppercase.
For the equations below, the angles are named with the Greek letters alpha (α), beta (β) and gamma (γ). Side a is opposite angle alpha, side b opposite angle beta, and side c opposite angle gamma.
The most commonly encountered formula for a triangle's area is the geometric one: half the product of the length of a side (b in the drawing) and the length of a line (called the altitude, labeled h in the drawing) perpendicular to that side and drawn from the opposite vertex. Let K = the area of the triangle.
The area can also be calculated from the lengths of the three sides alone (Heron's formula). Let s equal half the perimeter.
A circle can be inscribed in any triangle; its center is at the point where the lines bisecting the triangle’s angles meet. Its radius is given by (s as defined above):
Using s and r as defined above,
The most famous of triangle equations, the Pythagorean Theorem:
c² = a² + b²
where c is the hypotenuse, the side opposite the right angle.
An isosceles triangle is one in which two sides are the same length.
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Last revised: 12 August 2003.