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A square root of a number x is a number r which, when squared, becomes x: =. Every positive real number has two square roots, one positive and one negative. For example, the two square roots of 25 are 5 and −5. The positive square root is also known as the principal square root, and is denoted with a radical sign:
Square roots of negative numbers are called imaginary because in early-modern mathematics, only what are now called real numbers, obtainable by physical measurements or basic arithmetic, were considered to be numbers at all – even negative numbers were treated with skepticism – so the square root of a negative number was previously considered undefined or nonsensical.
An illustration of the complex plane. The imaginary numbers are on the vertical coordinate axis. Although the Greek mathematician and engineer Heron of Alexandria is noted as the first to present a calculation involving the square root of a negative number, [6] [7] it was Rafael Bombelli who first set down the rules for multiplication of complex numbers in 1572.
Every positive number x has two square roots: (which is positive) and (which is negative). The two roots can be written more concisely using the ± sign as . Although the principal square root of a positive number is only one of its two square roots, the designation "the square root" is often used to refer to the principal square root. [3] [4]
The radical symbol refers to the principal value of the square root function called the principal square root, which is the positive one. The two square roots of a negative number are both imaginary numbers, and the square root symbol refers to the principal square root, the one with a positive imaginary part. For the definition of the ...
The multiplicative inverse (reciprocal) of the square root of two is a widely used constant, with the decimal value: [20] 0.70710 67811 86547 52440 08443 62104 84903 92848 35937 68847 ... It is often encountered in geometry and trigonometry because the unit vector , which makes a 45° angle with the axes in a plane , has the coordinates
Although there are no real square roots of −1, the complex number i satisfies i 2 = −1, and as such can be considered as a square root of −1. [2] The only other complex number whose square is −1 is − i because there are exactly two square roots of any non‐zero complex number, which follows from the fundamental theorem of algebra.
By comparison, powers of two with negative exponents are fractions: for positive integer n, 2 −n is one half multiplied by itself n times. Thus the first few negative powers of 2 are 1 / 2 , 1 / 4 , 1 / 8 , 1 / 16 , etc. Sometimes these are called inverse powers of two because each is the multiplicative inverse of ...