Square (algebra)
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In algebra, the square of a number is that number multiplied by itself. To square a quantity is to multiply it by itself. Its notation is a superscripted "2"; a number x squared is written as x². Thus:
If x is a positive real number, the value of x² is equal to the area of a square of edge length x.
A positive integer that is the square of some other integer, for example 25 which is 5², is known as a square number, or more simply a square.
It is often also useful to note that the square of any number can be represented as the sum (for 0≤n)
- 1 + 1 + 2 + 2 + ... + (n − 1) + (n − 1) + n.
For instance, the square of 4 or 4² is equal to
- 1 + 1 + 2 + 2 + 3 + 3 + 4 = 16.
This is the result of adding a column and row of thickness 1 to the square graph of three (like a tic tac toe board). You add three to the side and four to the top to get four squared. This can also be useful for finding the square of a large number quickly. For instance, the square of
- 52² = 50² + 50 + 51 + 51 + 52 = 2500 + 204 = 2704.
In addition, it can be seen that another equivalent sum may be used to represent the square of a number. The square of a number N is the sum of the first N odd numbers. The square of 1 is 1; the square of 2 is
- 1 + 3 = 4;
the square of 7 is
- 1 + 3 + 5 + 7 + 9 + 11 + 13 = 49.
and so on. This, of course is the same as the previous sum method but with every two numbers following the initial number added to each other:
- 1 + ( 1 + 2 ) + ( 2 + 3 ) + ( 3 + 4 ) + ... = 1 + 3 + 5 + 7 + ...
The general term of the series 12 + 22 + 32 + 42 + ... + n2 is n(n + 1)(2n + 1) / 6. The first terms of this series (the Square pyramidal numbers) are :
0, 1, 5, 14, 30, 55, 91, 140, 204, 285, 385, 506, 650, 819, 1015, 1240, 1496, 1785, 2109, 2470, 2870, 3311, 3795, 4324, 4900, 5525, 6201... (sequence A000330 in OEIS)
[edit] Uses
Since the product of two real negative numbers is positive, and the product of two real positive numbers is also positive, it follows that no square number is negative. This has important consequences. It follows, in particular, that no square root can be taken of a negative number within the system of real numbers. This leaves a gap in the real number system that mathematicians fill by postulating complex numbers, beginning with the imaginary unit i, which by convention is one of the square roots of −1.
Squaring is also useful in statistics in determining the standard deviation of a set of values. The deviation of each value 
from the mean 
of the set is defined as the difference 
. These deviations are squared, then a mean is taken of the new set of numbers (each of which is positive). This mean is the variance, and its square root is the standard deviation. In finance, the volatility of a financial instrument is the standard deviation of its values.
