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In geometry, a **hypotenuse** is the side of a right triangle opposite the right angle.^{[1]} It is the longest side of any such triangle; the two other shorter sides of such a triangle are called *catheti* or *legs*. The length of the hypotenuse can be found using the Pythagorean theorem, which states that the square of the length of the hypotenuse equals the sum of the squares of the lengths of the two legs. Mathematically, this can be written as , where *a* is the length of one leg, *b* is the length of another leg, and *c* is the length of the hypotenuse.^{[2]}

For example, if one of the legs of a right angle has a length of 3 and the other has a length of 4, then their squares add up to 25 = 9 + 16 = 3 × 3 + 4 × 4. Since 25 is the square of the hypotenuse, the length of the hypotenuse is the square root of 25, that is, 5. In other words, if and , then .

Look up **ὑποτείνουσα** in Wiktionary, the free dictionary.

The word *hypotenuse* is derived from Greek ἡ τὴν ὀρθὴν γωνίαν __ὑποτείνουσα__ (sc. γραμμή or πλευρά), meaning "[side] __subtending__ the right angle" (Apollodorus),^{[3]} ὑποτείνουσα *hupoteinousa* being the feminine present active participle of the verb ὑποτείνω *hupo-teinō* "to stretch below, to subtend", from τείνω *teinō* "to stretch, extend". The nominalised participle, ἡ ὑποτείνουσα, was used for the hypotenuse of a triangle in the 4th century BCE (attested in Plato, *Timaeus* 54d). The Greek term was loaned into Late Latin, as *hypotēnūsa*.^{[4]}^{[better source needed]}^{[5]} The spelling in *-e*, as *hypotenuse*, is French in origin (Estienne de La Roche 1520).^{[6]}

In a right triangle, the hypotenuse is the side that is opposite the right angle, while the other two sides are called the *catheti* or *legs*.^{[7]} The length of the hypotenuse can be calculated using the square root function implied by the Pythagorean theorem. It states that the sum of the two legs squared equals the hypotenuse squared. In mathematical notation, with the respective legs labelled *a* and *b*, and the hypotenuse labelled *c*, it is written as . Using the square root function on both sides of the equation, it follows that

As a consequence of the Pythagorean theorem, the hypotenuse is the longest side of any right triangle; that is, the hypotenuse is longer than either of the triangle's legs. For example, given the length of the legs *a* = 5 and *b* = 12, then the sum of the legs squared is (5 × 5) + (12 × 12) = 169, the square of the hypotenuse. The length of the hypotenuse is thus the square root of 169, denoted , which equals 13.

The Pythagorean theorem, and hence this length, can also be derived from the law of cosines in trigonometry. In a right triangle, the cosine of an angle is the ratio of the leg adjacent of the angle and the hypotenuse. For a right angle *γ* (gamma), where the adjacent leg equals 0, the cosine of *γ* also equals 0. The law of cosines formulates that holds for some angle *θ* (theta). By observing that the angle opposite the hypotenuse is right and noting that its cosine is 0, so in this case *θ* = *γ* = 90°:

Many computer languages support the ISO C standard function hypot(*x*,*y*), which returns the value above.^{[8]} The function is designed not to fail where the straightforward calculation might overflow or underflow and can be slightly more accurate and sometimes significantly slower.

Some languages have extended the definition to higher dimensions. For example, C++17 supports ;^{[9]} this gives the length of the diagonal of a rectangular cuboid with edges *x*, *y*, and *z*. Python 3.8 extended to handle an arbitrary number of arguments. ^{[10]}

Some scientific calculators^{[which?]} provide a function to convert from rectangular coordinates to polar coordinates. This gives both the length of the hypotenuse and the angle the hypotenuse makes with the base line (*c _{1}* above) at the same time when given

By means of trigonometric ratios, one can obtain the value of two acute angles, and , of the right triangle.

Given the length of the hypotenuse and of a cathetus , the ratio is:

The trigonometric inverse function is:

in which is the angle opposite the cathetus .

The adjacent angle of the catheti is = 90° –

One may also obtain the value of the angle by the equation:

in which is the other cathetus.