first seen in print without the vinculum (the horizontal "bar" over the numbers inside the radical symbol) in the year 1525 in Die Coss by Christoff Rudolff , a German mathematician. The term surd traces back to al-Khwārizmī (c. 825), who referred to rational and irrational numbers as audible and inaudible , respectively. This later led to the Arabic word " أصم ‎" ( asamm , meaning "deaf" or "dumb") for irrational number being translated into Latin as "surdus" (meaning "deaf" or "mute"). Gerard of Cremona (c. 1150), Fibonacci (1202), and then Robert Recorde (1551) all used the term to refer to unresolved irrational roots . [ 5 ] The n th root of a number x , where n is a positive integer, is a number r whose n th power is x : Every positive real number x has a single positive n th root, which is written . For n equal to 2 this is called the square root and the n is omitted. The n th root can also be represented using exponentiation as x 1/n . For even values of n , positive numbers also have a negative n th root, while negative numbers do not have a real n th root. For odd values of n , every negative number x has a real negative n th root. For example, −2 has a real 5th root, but −2 does not have any real 6th roots. Every non-zero number x , real or complex ,