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In geometry, a **hypotenuse** is the longest side of a right-angled triangle, the side opposite of the right angle. The length of the hypotenuse of a right triangle 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 other two sides. For example, if one of the other sides has a length of 3 (when squared, 9) and the other has a length of 4 (when squared, 16), then their squares add up to 25. The length of the hypotenuse is the square root of 25, that is,5.

The word *hypotenuse* means essentially "length under", and derives from Latin *hypotēnūsa*, a transliteration of Ancient Greek *hypoteínousa* (*pleurā́* or *grammḗ*), the feminine present participle of *hypoteínō*, a combination of *hypó* ("under") and *teínō* ("I stretch" or "length").^{[1]}^{[2]} The word ὑποτείνουσα was used for the hypotenuse of a triangle by Plato in the Timaeus (dialogue) 54d and by many other ancient authors.

A folk etymology says that *tenuse* means "side", so *hypotenuse* means a support like a prop or buttress,^{[3]} but this is inaccurate.

Usually the length of the hypotenuse is calculated using the square root function derived from the Pythagorean theorem. Setting x = c_{1} and y = c_{2} to avoid subscripts:

In mathematical notation;

The length can also be derived from the law of cosines by setting *ɣ* to 90°:

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

Some scientific calculators 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

- The length of the hypotenuse equals the sum of the lengths of the orthographic projections of both catheti. And

- The square of the length of a cathetus equals the product of the lengths of its orthographic projection on the hypotenuse times the length of this.

**b² = a · m****c² = a · n**

- Also, the length of a cathetus
**b**is the proportional mean between the lengths of its projection**m**and the hypotenuse**a**.

**a/b = b/m****a/c = c/n**

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 , will be = 90° –

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

in which is the other cathetus.

- Cathetus
- Triangle
- Space diagonal
- Nonhypotenuse number
- Taxicab geometry
- Trigonometry
- Special right triangles
- Pythagoras

**^**Harper, Douglas. "hypotenuse".*Online Etymology Dictionary*.**^**u(potei/nw, u(po/, tei/nw, pleura/. Liddell, Henry George; Scott, Robert;*A Greek–English Lexicon*at the Perseus Project**^**Anderson, Raymond (1947).*Romping Through Mathematics*. Faber. p. 52.