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The square root of 3 is the positive real number that, when multiplied by itself, gives the number 3. It is more precisely called the principal square root of 3, to distinguish it from the negative number with the same property. It is denoted by

$$\sqrt{3}.$$

The first sixty significant digits of its decimal expansion are:

1.73205 08075 68877 29352 74463 41505 87236 69428 05253 81038 06280 5580... (sequence A002194 in OEIS)

The rounded value of 1.732 is correct to within 0.01% of the actual value. A close fraction is $$\tfrac{97}{56}$$ (1.7321 42857...).

The square root of 3 is an irrational number. It is also known as Theodorus' constant, named after Theodorus of Cyrene.

It can be expressed as the continued fraction [1; 1, 2, 1, 2, 1, 2, 1, ...] (sequence A040001 in OEIS), expanded on the right.

It can also be expressed by generalized continued fractions such as

$$[2; -4, -4, -4, ...] = 2 - \cfrac{1}{4 - \cfrac{1}{4 - \cfrac{1}{4 - \ddots}}}$$

which is [1;1, 2,1, 2,1, 2,1, ...] evaluated at every second term.

Proof of irrationality

This irrationality proof for the square root of 3 uses Fermat's method of infinite descent:

Suppose that √3 is rational, and express it in lowest possible terms (i.e., as a fully reduced fraction) as $$\frac{m}{n}$$ for natural numbers m and n. Then √3 can be expressed in lower terms as $$\frac{3n-m}{m-n}$$, which is a contradiction.  (The two fractional expressions are equal because equating them, cross-multiplying, and canceling like additive terms gives m^2=3n^2 and hence $$\tfrac{m}{n}=\sqrt{3}$$ , which is true by the premise. The second fractional expression for √3 is in lower terms since, comparing denominators, m-n<n since m<2n since$$\tfrac{m}{n}<2$$ since $$\sqrt{3}<2$$. And both the numerator and the denominator of the second fractional expression are positive since $$1<\tfrac{m}{n}<3 and \tfrac{m}{n}=\sqrt{3}.)$$

Geometry and trigonometry
The square root of 3 is equal to the length between parallel sides of a regular hexagon with sides of length 1.

If an equilateral triangle (equilateral polygon with three sides) with sides of length 1 is cut into two equal halves, by bisecting an internal angle across to make a right angle with one side, the right angle triangle's hypotenuse is length one and the sides are of length 1/2 and $$\sqrt{3}/2$$ . From this the trigonometric function tangent of 60 degrees equals \sqrt{3}, and the sine of 60° and the cosine of 30° both equal half of $$\sqrt{3}$$.

The square root of 3 also appears in algebraic expressions for various other trigonometric constants, including the sines of 3°, 12°, 15°, 21°, 24°, 33°, 39°, 48°, 51°, 57°, 66°, 69°, 75°, 78°, 84°, and 87°.

It is the distance between parallel sides of a regular hexagon with sides of length 1.

It is the length of the space diagonal of a unit cube.

The shape Vesica piscis has a major axis: minor axis ratio equal to the square root of three, this can be shown by constructing two equilateral triangles within it.

Other uses
Power engineering

In power engineering, the voltage between two phases equals $$\sqrt{3}$$ times the line to neutral voltage.

Square root of 2
Square root of 5

References

^ Grant, M.; Perella, M. (July 1999). "Descending to the irrational". Mathematical Gazette 83 (497): 263–267. doi:10.2307/3619054.
^ Julian D. A. Wiseman [Sin and Cos in Surds http://www.jdawiseman.com/papers/easymath/surds_sin_cos.html]

S., D.; Jones, M. F. (1968). "22900D approximations to the square roots of the primes less than 100". Mathematics of Computation 22 (101): 234–235. JSTOR 2004806.
Uhler, H. S. (1951). "Approximations exceeding 1300 decimals for \sqrt{3}, \frac{1}{\sqrt{3}}, \sin\left(\frac{\pi}{3}\right) and distribution of digits in them". Proc. Nat. Acad. Sci. U. S. A. 37: 443–447. PMC 1063398.
Wells, D. (1997). The Penguin Dictionary of Curious and Interesting Numbers (Revised ed.). London: Penguin Group. p. 23.