Squaring numbers

A convenient way to say that a number is to be multiplied by itself (for example 5×5) is to say that the number is squared. To help visualize this, picture a square which is 5 units long and 5 units wide. This square will then have an area of 5² or 25 square units. 5² can be read "five raised to the second" but is most commonly read "five squared."

Area of five rows by five columns = 5 × 5 = 5² = 25.

Cubing numbers

Similarly, a convenient way to say that a number is to be multiplied by itself and then itself again (for example 5×5×5) is to say that the number is cubed. To help visualize this, picture a cube which is 5 units long, 5 units wide, and 5 units high. This cube will then have a volume of 5³ or 125 cubic units. 5³ can be read "five to the third power" but is commonly read "five cubed."

Volume of a cube with height, width, and length of five = 5 × 5 × 5 = 5³ = 125.

Higher powers

Numbers higher than three may also be used as exponents, although there is no common term for numbers raised to a fourth power or higher. For example, 5⁴ = 5×5×5×5 = 625.

Properties of Exponents

There are several properties of exponents which are frequently used to manipulate and simplify algebraic and arithmetic expressions.

It follows immediately that

${\displaystyle x^{m}\cdot x^{n}=\underbrace {x\cdot x\cdots \cdot x} _{m\,factors}\cdot \underbrace {x\cdot x\cdots \cdot x} _{n\ factors}=\underbrace {x\cdot x\cdots \cdot x} _{m+n\ factors}=x^{m+n}}$

(1)

${\displaystyle (xy)^{n}=x^{n}y^{n}}$

(2)

and

${\displaystyle (x^{m})^{n}=x^{mn}}$

(3)

These can be extended to multiple factors

${\displaystyle {\begin{aligned}x^{m_{1}}x^{m_{2}}x^{m_{3}}\cdots x^{m_{n}}&=x^{m_{1}+m_{2}+m_{3}+\,\cdots \,+m_{n}}\\(x_{1}x_{2}x_{3}\cdots x_{n})^{m}&=x_{1}^{m}x_{2}^{m}x_{3}^{m}\cdots x_{n}^{m}\\(\cdots (((x^{m_{1}})^{m_{2}})^{m_{3}})\cdots )^{m_{n}}&=x^{m_{1}m_{2}m_{3}\cdots \,m_{n}}\end{aligned}}}$

Alternatively, we can use first property to obtain

${\displaystyle x^{0}=x^{1-1}=x^{1}\cdot x^{-1}\implies x^{-1}=x^{0}/x^{1}=1/x}$

(3)

and use others to write down

${\displaystyle {\dfrac {x^{m}}{x^{n}}}=x^{m-n}\quad {\text{and}}\quad \left({\dfrac {x}{y}}\right)^{n}={\dfrac {x^{n}}{y^{n}}}}$

(4)

Bases and exponents of one and zero

Any number raised to an exponent of one equals itself. So, for example, 5¹ = 5.

Any non-zero number raised to an exponent of zero equals one. So, for example, 5⁰ = 1.

Zero raised to any positive exponent is still zero. So, for example, 0⁵ = 0.

One raised to any exponent is still one. So, for example, 1⁵ = 1.

Zero raised to an exponent of zero is not defined.

Fractional powers

A base may also be raised to a fractional, or decimal power. This will be covered later in the lesson.

Square root

The inverse operation of squaring a number is taking the square root of that number. So, for example, the square root of 25 is the number which must be multiplied by itself to equal 25. In this case, the answer is 5. There are two types of notation used here:

${\displaystyle {\sqrt {25}}=25^{1/2}=5}$

Note, however, that most square roots don't yield integers, and many don't even produce rational numbers.

Cube root

The inverse operation of cubing a number is taking the cube root of that number. So, for example, the cube root of 125 is the number which must be multiplied by itself and then multiplied by itself again to equal 125. In this case, the answer is 5. There are two types of notation used here:

${\displaystyle {\sqrt[{3}]{125}}=125^{1/3}=5}$

Note, however, that most cube roots don't yield integers, and many don't even produce rational numbers.

Higher roots

Numbers higher than three may also be used as roots, although there is no common term for fourth roots or higher. For example:

${\displaystyle {\sqrt[{4}]{625}}=625^{1/4}=5}$

Note, however, that most higher roots don't yield integers, and many don't even produce rational numbers.

Fractions as exponents

Other (non-unit) fractions may also be used as exponents. In this case, the base number may be raised to the power of the numerator (top number in the fraction) then the denominator (bottom number) may be used to take the root. For example:

${\displaystyle 8^{2/3}={\sqrt[{3}]{8^{2}}}={\sqrt[{3}]{64}}=4}$

Alternatively, you can take the root first and then apply the power:

${\displaystyle 8^{2/3}=({\sqrt[{3}]{8}})^{2}=2^{2}=4}$

Decimal exponents

Any fractional exponent can also be expressed as a decimal exponent. For example, a square root may also be written as:

${\displaystyle 25^{1/2}=25^{0.5}=5\;}$

Also, decimals which can't be expressed as a fraction (irrational numbers) may be used as exponents:

${\displaystyle 5^{3.1415926...}=156.99...\;}$

Such problems aren't easy to solve by hand using basic math skills, but the answer can be estimated manually. In this case, since 3.1415926 is between 3 and 4 (and considerable closer to 3), we know that the answer will be between 5^3 (or 125) and 5^4 (or 625), and considerable closer to 125.

Negative exponents

A negative exponent simply means you take the reciprocal (one over the number) of the base first, then apply the exponent:

${\displaystyle 25^{-0.5}=1/25^{0.5}=1/5\;}$

Alternatively, you can first apply the exponent (ignoring the sign), then take the reciprocal:

${\displaystyle 25^{-0.5}=5^{-1}=1/5\;}$

Fractions as bases

When a fraction is raised to an exponent, both the numerator and denominator are raised to that exponent:

${\displaystyle (5/6)^{2}=5^{2}/6^{2}=25/36\;}$

Fractions may also be used for both the base and exponent:

${\displaystyle (25/36)^{1/2}=25^{1/2}/36^{1/2}=5/6\;}$

In addition, negative fractional exponents may be used, taking the reciprocal of the base, as always, to find the solution.

Negative bases

Negative bases can be handled normally for integer powers:

${\displaystyle (-5)^{2}=(-5)\times (-5)=25\;}$

${\displaystyle (-5)^{3}=(-5)\times (-5)\times (-5)=-125\;}$

${\displaystyle (-5)^{4}=(-5)\times (-5)\times (-5)\times (-5)=625\;}$

${\displaystyle (-5)^{5}=(-5)\times (-5)\times (-5)\times (-5)\times (-5)=-3125\;}$

Note that negative bases raised to even powers produce positive results, while negative bases raised to odd powers produce negative results.

Be careful with negative signs. Since -5 = -1×5, there is a difference between ${\displaystyle -5^{2}}$ and ${\displaystyle (-5)^{2}}$. The former means the negative of 5 times 5, whereas the latter
means -5 squared. In other words,

${\displaystyle -5^{2}=-1\times 5^{2}=-1\times (5\times 5)=-1\times 25=-25}$

but

${\displaystyle (-5)^{2}=(-5)\times (-5)=25}$

Roots and fractional/decimal powers are a bit trickier. Odd roots work out fine:

${\displaystyle (-125)^{1/3}=-5\;}$

Even roots, however, have no real solution:

${\displaystyle (-25)^{1/2}=?\;}$

${\displaystyle (-625)^{1/4}=?\;}$

Note that there is no real number, when multiplied by itself, which will produce -25, because 5×5 = 25 and -5×-5 = 25. There is actually a solution, called an imaginary number, but that won't be discussed until later lessons.

Principal root

Note that, since both 5×5 = 25 and -5×-5 = 25, when we are asked to take the square root of 25 there are, in fact, two valid answers, 5 and -5. Actually, any even root of a positive number will have two solutions, one positive and one negative. This may seem unusual, but, in higher mathematics, problems often have multiple solutions.

However, for many problems, only the positive value seems to physically work. For example, if we are asked to figure the length of the sides of a square yard which has an area of 25 square units, only 5 units on a side works. If we said "each side can also have a length of -5 units", that doesn't make any sense. For this reason, the positive solution is called the principal root, and, depending on the question, may be the only desired answer. In cases where either answer is valid, it is sometimes written as ±5 (read as "plus or minus five").