If a square has a side of length 7 feet, then it has an area of 72, or 49 square feet. If a square has an area of 36 square feet, then the length of its side is 6 feet. If we know the length of a side, then we square it to find the area. If we know the area, then we must undo the process of squaring to find the length of a side. Undoing the process of squaring is called taking the square root.
Because 32 = 9, 23 = 8, and (-2)4 = 16, we say that 3 is a square root of 9, 2 is the cube root of 8, and -2 is a fourth root of 16. In general, undoing an nth power is referred to as taking an nth root.
The number b is an nth root of a if bn = a.
Both 3 and -3 are square roots of 9 because 32 = 9 and (-3)2 = 9. Because 24 = 16 and (-2)4 = 16, there are two real fourth roots of 16: 2 and -2. If n is a positive even integer and a is any positive real number, then there are two real nth roots of a. We call these roots even roots. The positive even root of a positive number is called the principal root. The principal square root of 9 is 3, and the principal fourth root of 16 is 2. When n is even, the exponent 1/n is used to indicate the principal nth root. The principal nth root can also be indicated by the radical symbol .
Exponent 1/n When n Is Even
If n is a positive even integer and a is a positive real number, then a1/n denotes the positive real nth root of a and is called the principal nth root of a.
An exponent of n indicates nth power, an exponent of -n indicates the reciprocal of the nth power, and an exponent of 1/n indicates nth root.
We will see later that choosing 1/n to indicate nth root fits in nicely with the rules of exponents that we have already studied.
Finding even roots
Evaluate each expression.
a) Because 22 = 4, we have 41/2 = 2. Note that 41/2 ≠ -2.
b) Because 24 = 16, we have 161/4 = 2. Note that 161/4 ≠ -2.
c) Following the accepted order of operations from Chapter 1, we find the root first and then take the opposite of it. Because 34 = 81, we have 811/4 = 3 and -811/4 = -3.
d) Because , we have .
Note that 23 = 8 but (-2)3 = -8. The cube root of 8 is 2, and the cube root of -8 is -2. If n is a positive odd integer and a is any real number, then there is only one real nth root of a. We call this root an odd root.
Exponent 1/n When n Is Odd
If n is a positive odd integer and a is any real number, then a1/n denotes the real nth root of a.
Finding odd roots
Evaluate each expression.
a) Because 23 = 8, we have 81/3 = 2.
b) Because (-3)3 = -27, we have (-27)1/3 = -3.
c) Because 25 = 32, we have -321/5 = -2.
We do not allow 0 as the base when we use negative exponents because division by zero is undefined. However, positive powers of zero are defined, and so are roots of zero; for example, 04 = 0, and so 01/4 = 0.
nth Root of Zero
If n is a positive integer, then 01/n = 0.
An expression such as (-9)1/2 is not included in the definition of roots because there is no real number whose square is -9. The definition of roots does not include an even root of any negative number because no even power of a real number is negative.
The expression 31/2 represents the unique positive real number whose square is 3. Because there is no rational number that has a square equal to 3, the number 31/2 is an irrational number. If we use a calculator, we find that 31/2 is approximately equal to the rational number 1.732. Because the square root of 3 is not a rational number, the simplest representation for the exact value of the square root of 3 is 31/2.