Definitions of surds: A root of a positive real quantity is called a surd if its value cannot be exactly determined. It is a number that can’t be simplified to remove a square root (or cube root etc). For example, each of the quantities √3, ∛7, ∜19, (16)^2/5 etc. is a surd.

**Equiradical Surds**

If two or more surds are of the same order they are said to be equiradical.

Surds are not equiradical when their surd indices are different.

Thus, √5, √7, 2√5, √x and 10^{1/2} are equiradical surds.

But √2, ∛7, ∜6 and 9^2/5 are not equiradical.

Note: Non-equiradical surds can be reduced to equiradical surds.

Thus, non-equiradical surds √3, ∛3, ∜3 become ^{12}√729, ^{12}√81, ^{12}√27 respectively when they are reduced to equiradical surds.

From the definition, it is evident that a surd is an incommensurable quantity, although its value can be determined to any degree of accuracy. It should be noted that quantities √9, ∛64, ∜(256/625) etc. expressed in the form of surds are commensurable quantities and are not surds (since √9 = 3, ∛64 = 4, ∜(256/625) = 4/5 etc.). In fact, any root of an algebraic expression is regarded as a surd.

Note: All surds are irrationals but all irrational numbers are not surds. Irrational numbers like π and e, which are not the roots of algebraic expressions, are not surds.

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