# 3. Simplest Radical Form

Before we can simplify radicals, we need to know some rules about them. These rules just follow on from what we learned in the first 2 sections in this chapter, Integral Exponents and Fractional Exponents.

Expressing in simplest radical form just means simplifying a radical so that there are no more square roots, cube roots, 4th roots, etc left to find. It also means removing any radicals in the denominator of a fraction.

## Laws of Radicals

Let's take the positive case first.

### n-th root of a Positive Number to the Power n

We met this idea in the last section, Fractional Exponents. Basically, finding the n-th root of a (positive) number is the opposite of raising the number to the power n, so they effectively cancel each other out. These 4 expressions have the same value:

root(n)(a^n)=(root(n)a)^n=root(n)((a^n))=a

The 2nd item in the equality above means:

"take the n-th root first, then raise the result to the power n"

The 3rd item means:

"raise a to the power n then find the n-th root of the result"

Both steps lead back to the a that we started with.

For the simple case where n = 2, the following 4 expressions all have the same value:

sqrt(a^2)=(sqrt(a))^2=sqrt((a^2))=a

For example, if a = 9:

sqrt(9^2)=(sqrt(9))^2=sqrt((9^2))=9

The second item means: "Find the square root of 9 (answer: 3) then square it (answer 9)".

The 3rd item means: "Square 9 first (we get 81) then find the square root of the result (answer 9)".

In general we could write all this using fractional exponents as follows:

root(n)(a^n)=(a^(1//n))^n=(a^n)^(1//n)=a

Yet another way of thinking about it is as follows:

(a^(1/n))^n=a^((1/nxxn))=a

### n-th root of a Negative Number to the Power n

We now consider the above square root example if the number a is negative.

For example, if a = -5, then:

sqrt((-5)^2)=sqrt(25)=5

A negative number squared is positive, and the square root of a positive number is positive.

In general, we write for a, a negative number:

sqrt((a)^2)=|a|

Notice I haven't included this part: (sqrt(a))^2. In this case, we would have the square root of a negative number, and that behaves quite differently, as you'll learn in the Complex Numbers chapter later.

Continued below

### The Product of the n-th root of a and the n-th root of b is the n-th root of ab

root(n)axxroot(n)b=root(n)(ab)

Example:

root(4)7xxroot(4)5=root(4)(7xx5)=root(4)35

We could write "the product of the n-th root of a and the n-th root of b is the n-th root of ab" using fractional exponents as well:

a^(1//n)xxb^(1//n)=(ab)^(1//n)

### The m-th Root of the n-th Root of the Number a is the mn-th Root of a

root(m)(root(n)a)=root(m\ n)a

We could write this as:

(a^(1//n))^(1//m)=(a)^(1//(mn))

Example:

root(4)(root(3)5)=root(12)5

This has the same meaning:

(5^(1//3))^(1//4)=(5)^(1//(12))

In words, we would say: "The 4th root of the 3rd root of 5 is equal to the 12th root of 5".

### The n-th Root of a Over the n-th Root of b is the n-th Root of a/b

root(n)a/root(n)b=root(n)(a/b)(b ≠ 0)

Example:

root(3)375/root(3)3=root(3)(375/3)=root(3)125=5

If we write the our general expression using fractional exponents, we have:

a^(1//n)/b^(1//n)=(a/b)^(1//n) (b ≠ 0)

### Mixed Examples

Simplify the following:

(a) root(5)(4^5)

(b) root(3)2root3(3)

(c) root(3)sqrt5

(d) sqrt7/sqrt3

### Simplest Radical Form Examples

In these examples, we are expressing the answers in simplest radical form, using the laws given above.

(a) sqrt72

(b) sqrt(a^3b^2)

(c) root(3)40

(d) root(5)(64x^8y^(12))

### Exercises

Simplify:

Q1 sqrt(12ab^2)

Q2 root(4)(64r^3s^4t^5

Q3 sqrt(x/(2x+1)

This one requires a special trick. To remove the radical in the denominator, we need to multiply top and bottom of the fraction by the denominator.

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