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Math II · Mathematics
Numbers · Powers, Roots and Logarithms

Properties of Exponents

A chain message hits your group chat: "Forward this to 2 people!" Each person who gets it forwards it to 2 new people, round after round. Could it reach more than a billion people in 30 rounds, without you multiplying 2 by itself 30 times? Exponent rules let you answer that in a few lines. You will find out by the end of this page.

Same base: the three core rules

You already know that $2^3 \cdot 2^5 = 2^8$. The real skill is spotting which rule to use inside a longer expression, with negative exponents, variables, or hidden in an equation. Let's practice exactly that.

Three exponent rules for powers with the same base: add exponents when multiplying, subtract when dividing, multiply when raising a power to a power. aᵐ · aⁿ = aᵐ⁺ⁿ same base: add exponents aᵐ ÷ aⁿ = aᵐ⁻ⁿ same base: subtract exponents (aᵐ)ⁿ = aᵐⁿ power of a power: multiply exponents

The rules work for negative bases too. Just keep the parentheses: $((-7)^3)^2 = (-7)^6$.

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Express as a single power with base $5$ by applying the properties of powers: $\frac{5^4 \cdot 5^{-2}}{5^3}$.

Express as a single power with base $5$ by applying the properties of powers: $\frac{5^4 \cdot 5^{-2}}{5^3}$.

Solve
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Reduce to a single power using the appropriate properties: $\left((-7)^3\right)^{-4} \cdot (-7)^5$.

Reduce to a single power using the appropriate properties: $\left((-7)^3\right)^{-4} \cdot (-7)^5$.

Solve

Negative exponents

Look at the powers of $2$ going down. Each step to the right divides by $2$. Keep going past $2^0 = 1$ and you land on fractions.

Powers of 2 from 2 cubed down to 2 to the negative 3, with each step dividing by 2. 2³ 2² 2¹ 2⁰ 2⁻¹ 2⁻² 2⁻³ 8 4 2 1 1/2 1/4 1/8 each step to the right divides by 2, so the pattern continues below zero
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Calculate the value of the following power and express the result as an irreducible fraction: $\left(\frac{2}{3}\right)^{-3}$.

Calculate the value of the following power and express the result as an irreducible fraction: $\left(\frac{2}{3}\right)^{-3}$.

Solve
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Express the number $\frac{1}{128}$ as a power with base $2$ using a negative exponent.

Express the number $\frac{1}{128}$ as a power with base $2$ using a negative exponent.

Solve

Same exponent, different bases

The first three rules need the same base. A fourth rule needs the same exponent.

Check that the product of powers with the same exponent equals the power of the product: 2 squared times 3 squared equals 6 squared, both 36. 2² · 3² = 4 · 9 = 36 (2 · 3)² = 6² = 36 same exponent: multiply the bases
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Write as a single power by applying the properties of operations with powers of the same exponent: $15^4 : 3^4 \cdot 2^4$.

Write as a single power by applying the properties of operations with powers of the same exponent: $15^4 : 3^4 \cdot 2^4$.

Solve
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State whether the following statement is true or false, justifying your answer using the properties of powers: "Double $2^8$ is $4^8$".

State whether the following statement is true or false, justifying your answer using the properties of powers: "Double $2^8$ is $4^8$".

Solve

Simplifying expressions with variables

Variables work exactly like numbers. In this page every variable in a denominator is assumed to be nonzero. The usual goal is an answer with positive exponents only.

A negative exponent moves its factor across the fraction bar: a to the negative 3 times b squared over c to the negative 1 equals b squared times c over a cubed. a⁻³ · b² c⁻¹ = b² · c a³ a negative exponent moves the factor across the fraction bar
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Simplify the following algebraic expression leaving the result with positive exponents: $\frac{x^3 \cdot y^{-2}}{x^{-1} \cdot y^4}$.

Simplify the following algebraic expression leaving the result with positive exponents: $\frac{x^3 \cdot y^{-2}}{x^{-1} \cdot y^4}$.

Solve
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Simplify the following algebraic expression fully, using the properties of powers and leaving the result without negative exponents:…

Simplify the following algebraic expression fully, using the properties of powers and leaving the result without negative exponents: $\left( \frac{a^{-2} \cdot b^3}{a \cdot b^{-1}} \right)^{-2}$.

Solve
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Simplify the following algebraic expression, where $x$ and $y$ are non-zero real numbers and $a, b$ are positive integers:…

Simplify the following algebraic expression, where $x$ and $y$ are non-zero real numbers and $a, b$ are positive integers:

$\left( \frac{x^{a-b} \cdot y^{b+a}}{x^{a+b} \cdot y^{b-a}} \right)^{-1}$

Apply the properties of powers with the same base and express the final result using only positive exponents.

Solve

Comparing numbers with a common base

Rules 1–3 need the same base, so when bases differ, rewrite them. The trick is to turn each base into a power of a smaller number, usually a prime.

To compare 2 to the 45th with 4 to the 20th, rewrite 4 to the 20th as 2 to the 40th and compare the exponents. 4²⁰ = (2²)²⁰ = 2⁴⁰ 2⁴⁵ versus 2⁴⁰ same base 2, so the larger exponent wins: 45 > 40, so 2⁴⁵ > 4²⁰

For bases greater than $1$, a larger exponent means a larger number.

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Without using a calculator, determine which of the following quantities is greater: $3^{50}$ or $9^{20}$. Justify your answer by transforming one of the powers so that…

Without using a calculator, determine which of the following quantities is greater: $3^{50}$ or $9^{20}$. Justify your answer by transforming one of the powers so that both have the same base.

Solve
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Simplify the following expression by factorizing the bases first and applying the properties of powers. Express the result as a product of powers with prime bases:…

Simplify the following expression by factorizing the bases first and applying the properties of powers. Express the result as a product of powers with prime bases: $\frac{18^2 \cdot 12^{-3}}{24^{-1} \cdot 9^3}$.

Solve

Common factors and checking your work

The product rules do not apply to sums or differences. But if the exponents contain a variable, you can factor out the smallest power.

Factoring the common power 2 to the n out of 2 to the n plus 3 plus 2 to the n. 2ⁿ⁺³ + 2ⁿ = 2ⁿ · 2³ + 2ⁿ · 1 = 2ⁿ · (8 + 1) = 9 · 2ⁿ write the larger exponent as 2ⁿ times a number, then factor out 2ⁿ
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Simplify the following expression in which the exponents depend on the parameter $n$, taking out a common factor if necessary to be able to operate:…

Simplify the following expression in which the exponents depend on the parameter $n$, taking out a common factor if necessary to be able to operate: $\frac{3^{n+2} - 3^n}{3^{n+1} + 3^n}$.

Solve
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A student has shown the following algebraic working to simplify an expression:…

A student has shown the following algebraic working to simplify an expression:

$\frac{10^4 \cdot 5^{-2}}{2^4 \cdot 5^2} = \frac{(2 \cdot 5)^4 \cdot 5^{-2}}{2^4 \cdot 5^2} = \frac{2^4 \cdot 5^4 \cdot 5^{-2}}{2^4 \cdot 5^2} = 5^{4 - 2 - 2} = 5^0 = 1$

Identify if there is any error in their reasoning, explain it in detail and provide the correct result by strictly applying the properties of powers.

Solve

Solving exponential equations

An exponential equation has the unknown in an exponent. If you can write both sides as powers of the same base, then the exponents must be equal. This works because for a base greater than $1$, different exponents give different values. Then you solve the resulting linear equation.

The curve $y = 2^x$ meets the line $y = 8$ at only one point, $x = 3$. That is why $2^x = 2^3$ forces $x = 3$.

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Find the value of the exponent $x$ so that the following equality holds: $2^x \cdot 2^3 = 2^{-1}$.

Find the value of the exponent $x$ so that the following equality holds: $2^x \cdot 2^3 = 2^{-1}$.

Solve
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Solve the following exponential equation by expressing all terms as powers of the same base and operating with their exponents: $4^x \cdot 2^{x+1} = \frac{1}{8}$.

Solve the following exponential equation by expressing all terms as powers of the same base and operating with their exponents: $4^x \cdot 2^{x+1} = \frac{1}{8}$.

Solve
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Find the values of $x$ and $y$ that simultaneously satisfy the following equalities, using the properties of powers to transform the system into a system of linear…

Find the values of $x$ and $y$ that simultaneously satisfy the following equalities, using the properties of powers to transform the system into a system of linear equations:

$2^x \cdot 4^y = 256$

$\frac{3^{2x}}{9^y} = 27$

Solve

Growth and decay

When a quantity is multiplied by the same growth factor $b$ every time period, after $t$ periods you get

$$\text{amount} = (\text{start}) \cdot b^{t}.$$

Growth has $b > 1$. Decay has $0 < b < 1$, and a negative exponent can show up naturally: multiplying by $\frac{1}{2}$ repeatedly is the same as $2^{-t}$.

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A culture of bacteria in a biology laboratory doubles every hour. If initially there are $1000$ bacteria, express as a product with a power the number of bacteria there…

A culture of bacteria in a biology laboratory doubles every hour. If initially there are $1000$ bacteria, express as a product with a power the number of bacteria there will be after $12$ hours and calculate its final value.

Solve
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A radioactive substance reduces its mass to a third every day that passes. If today we have a sample of $81$ ounces, express using a power with base $3$ the amount of…

A radioactive substance reduces its mass to a third every day that passes. If today we have a sample of $81$ ounces, express using a power with base $3$ the amount of substance that will remain in $6$ days and calculate the result.

Solve
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A capital is invested at an annual compound interest such that each year the money is multiplied by $1,05$. If the initial investment is $\$2000…

A capital is invested at an annual compound interest such that each year the money is multiplied by $1,05$. If the initial investment is $\$2000$, write the expression in the form of a power that calculates the capital after $t$ years. How much money will there be after $4$ years?

Solve

Doubling over time and estimating huge numbers

If something doubles every $k$ time units, then after $t$ units it has been multiplied by $2^{t/k}$. For example, "doubling every $2$ years" gives $N = N_0 \cdot 2^{t/2}$. If $t = 6$ years, that is $2^3 = 8$ times as much, not $2^6$.

To estimate big powers of $2$ without a calculator, use the handy approximation

$$2^{10} = 1{,}024 \approx 10^3.$$

Back to the chain message: in round $n$ there are $2^n$ new recipients. In round $30$ that is $2^{30} \approx 10^9$, more than a billion people, which is more than the population of the United States many times over. Powers grow fast!

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Moore's empirical law suggests that the number of transistors in a microprocessor doubles approximately every $2$ years. If in the year 1970 a processor had $2000$…

Moore's empirical law suggests that the number of transistors in a microprocessor doubles approximately every $2$ years. If in the year 1970 a processor had $2000$ transistors, express the number of transistors in the year $1970 + t$ using a power with base $2$. Use this property to estimate how many transistors a processor would have in the year 2010.

Solve
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Imagine you have a sheet of paper $0,1\text{ mm}$ thick. Every time you fold the sheet in half on itself, its thickness doubles. Express the thickness of the sheet after…

Imagine you have a sheet of paper $0,1\text{ mm}$ thick. Every time you fold the sheet in half on itself, its thickness doubles. Express the thickness of the sheet after $n$ folds using a power. If the average distance from the Earth to the Moon is approximately $384\,000\text{ km}$, set up an equation using powers of $2$ to estimate how many folds would be needed for the thickness of the paper to reach the Moon (you can estimate the value knowing that $2^{10} \approx 10^3$).

Solve

Scaling: how size changes area, volume and light

Powers also describe how things change when you resize them. If you multiply lengths by $k$, then areas are multiplied by $k^2$ and volumes by $k^3$. This is the same-exponent rule: $(ka)^2 = k^2a^2$.

Squares with side 1, 2 and 3 have areas 1, 4 and 9: doubling the side multiplies the area by 4. side 1 side 2 side 3 area 1 area 4 area 9 side × k area × k² volume × k³

Some quantities shrink with distance. The light from a star (or a lamp) is inversely proportional to the square of the distance: if you are $d$ times farther away, the intensity is multiplied by $d^{-2} = \dfrac{1}{d^2}$.

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A packaging company manufactures cubic boxes. If they decide to triple the edge length of a standard box, by what factor will its volume be multiplied? Express your…

A packaging company manufactures cubic boxes. If they decide to triple the edge length of a standard box, by what factor will its volume be multiplied? Express your reasoning using the properties of the power of a product.

Solve
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In astrophysics, the intensity of the light we receive from a star is inversely proportional to the square of the distance we are from it. If we move to a distance $4$…

In astrophysics, the intensity of the light we receive from a star is inversely proportional to the square of the distance we are from it. If we move to a distance $4$ times greater from the star, express using a power with a negative exponent the factor by which the original light intensity is multiplied. If the initial intensity was $I_0$, what will the new intensity be in terms of $I_0$?

Solve
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The Sierpinski carpet is a fractal constructed from a square. In each iteration, each solid square is divided into $9$ equal squares and the central one is removed. If…
n = 0 n = 1 n = 2

The Sierpinski carpet is a fractal constructed from a square. In each iteration, each solid square is divided into $9$ equal squares and the central one is removed. If the initial area ($n=0$) is $1\text{ m}^2$, in the first iteration the remaining area is $\frac{8}{9}\text{ m}^2$. Express the remaining area after $n$ iterations using powers. Calculate the area that will remain after $5$ iterations.

Solve

Summary

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