katetu

Katetu is a term used in geometry to denote a leg of a right triangle—the two sides that form the right angle. Each right triangle has exactly two catetu, paired with a single hypotenuse, which is opposite the right angle.

In the standard notation, the lengths of the cateti are usually labeled a and b, while the hypotenuse is c. The relationship among them is given by the Pythagorean theorem: a^2 + b^2 = c^2. The cateti are the sides that directly determine the size of the hypotenuse through this equation.

For any acute angle in a right triangle, one cathetus is opposite the angle and the other is adjacent. If we denote the acute angle by α, and the opposite leg by a with the adjacent leg b and hypotenuse c, then the basic trigonometric ratios apply: sin α = a/c, cos α = b/c, tan α = a/b. These relationships show how the cateti determine the values of the triangle’s angles and its trigonometric functions.

In coordinate geometry, the cateti correspond to perpendicular directions, such as horizontal and vertical distances, when the right angle is aligned with coordinate axes. This makes cateti a useful concept in problems involving distances, projections, and right-triangle constructions.

Terminology varies by language. The English term is cathetus (plural catheti). The Indonesian term katetu is used in some texts to refer to the same concept, though many sources simply use “leg” for clarity. See also right triangle, hypotenuse, and the Pythagorean theorem.