Take a sheet of paper 0.1 mm thick and fold it in half: 2 layers. Fold again: 4. Again: 8. How many folds would it take for the stack to reach the Moon, 3,84,400 km away?
It sounds impossible, but every fold doubles the stack. After n folds there are layers, and numbers written this way, with a small raised index, grow faster than you expect. By Back to the paper you will answer the question with the laws of this unit, in two lines, and see why 42 is enough. On the way you will find out what , and mean.
Real paper stops long before that: in 2002 Britney Gallivan folded a 1.2 km strip of paper in half 12 times, the record then. Sources at the end of the unit.
12.0 Review
The distance from the Earth to the Sun is about 1,49,60,00,00,000 m, and light travels 30,00,00,000 m in one second. Discuss in your class:
- a.How can we write the distance from the Earth to the Sun in a short form?
- b.How can we write the speed of light in a short form?
- c.Which format is used to write large numbers in a short form?
Short forms are not only shorter: they make sums easy. How long does sunlight take to reach us?
That is about 8 minutes 19 seconds. What it means: the Sun you see is the Sun as it was eight minutes ago. Notice the step : the indices were simply subtracted. Why that works is the subject of this unit.
12.1 Indices
When the same number or variable is multiplied by itself several times, we write it once, as the base, with a small raised number, the index (plural indices), which counts how many times it is multiplied:
is read “a to the power n”, and the whole is called a power of a. The index is also called the exponent. So : the base is 2, the index 5, and 32 is the value.
In , which is the base and which is the index?
Pick an answer to see why.
Which is bigger, or ?
Pick an answer to see why.
12.2 Laws of indices
The laws below are not rules to memorize blindly: each one is just counting how many times the base is multiplied. Throughout, the base a (or b) is not zero.
a. Multiplication law
Three a’s and then two more: five in all. In general, m of them and then n more:
To multiply powers of the same base, add the indices. The same holds for more than two: .
b. Division law
Each a below cancels one a above; 5 − 3 = 2 are left. In general:
To divide powers of the same base, subtract the indices. When there are more a’s below, what is left stays below the line: .
c. Power law
Two copies of three a’s: six. In general, n copies of m a’s:
A power of a power: multiply the indices. Two more follow from the same counting, because the order of multiplying doesn’t matter:
So and .
What is ?
Pick an answer to see why.
d. Law of negative indices
In the division law, try . By canceling it is ; by subtracting the indices it is . For the law to give the right answer every time, the two must be the same thing:
A negative index means “one over”: . It does not make the number negative.
e. Law of zero index
Now try . Every a cancels, so it is 1; and by subtracting, it is . So
for every base that isn’t 0: , , , . The same comes from the pattern below: each step to the left divides by the base, so the step left of is a ÷ a = 1.
What is ?
Pick an answer to see why.
f. Root law
What could mean? If the laws are to hold, then
So is the number that, multiplied by itself, gives a: the square root. In the same way, three copies of multiply to a, so , the cube root. In general:
In , the denominator is the root and the numerator the power: . Take the root first when it is a whole number: the numbers stay small.
What is ?
Pick an answer to see why.
12.3 Simplification of problems related to indices
To simplify, the plan is nearly always the same: write every number as a power of a prime, every bracket as one power, then add or subtract the indices. Prime bases worth knowing by heart: , , , , , ; , , , ; , .
When the indices are letters, the same laws turn each bracket into one power of x; then the indices are added, and the identities of Unit 9 often make the sum 0.
Back to the paper
After 42 folds the sheet has layers. A handy fact: . So
Each layer is m thick, so the stack is
that is about 4,00,000 km (exactly, 4,39,805 km). 42 folds take the stack past the Moon, 3,84,400 km away. 41 folds give half of that, about 2,20,000 km: not enough.
What it means: each fold adds only 1 to the index, but doubles the thickness. Growth like this, where a quantity is multiplied by the same number at each step, is called exponential growth: it is how money grows at compound interest, how bacteria multiply, and how a rumor or a virus can spread.
Exercise 12.1
Question 1
Find the value of .
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Find the value of .
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Find the value of .
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Find the value of .
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Find the value of .
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Find the value of .
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Find the value of .
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Find the value of .
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Find the value of .
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Find the value of .
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Find the value of .
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Question 2
Simplify .
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Simplify .
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Simplify .
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Simplify .
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Simplify .
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Simplify .
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Simplify .
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Simplify .
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Question 3
Find the value of .
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Find the value of .
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Find the value of .
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Find the value of .
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Find the value of .
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Question 4
Simplify .
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Simplify .
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Simplify .
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Simplify .
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Simplify .
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Simplify .
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Question 5
Simplify .
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Simplify .
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Simplify .
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Simplify .
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Simplify .
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Simplify .
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Simplify .
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Simplify .
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Question 6
Prove that .
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Prove that .
The textbook prints the roots as the c-th, a-th and b-th. Then the index is , which is not 0 in general (a = 1, b = 2, c = 3 gives ), so the book’s numbers don’t work. With square roots for all three, as the Nepali edition prints it and as here, the statement is true.
Answer
Prove that .
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Prove that .
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Prove that .
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Prove that .
The textbook prints and . These don’t factorize, and then the two sides are not equal (x = 3, y = 2, a = 1, b = 2 gives about 3.19 against 3.375), so the book’s numbers don’t work. With minus signs, as the Nepali edition prints them and as in Example 5, they are.
Answer
Prove that .
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Question 7
If , prove that .
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Question 8
If , prove that .
The textbook prints the condition as . Then the left side comes to , not 1, so the book’s numbers don’t work. Without the 2, as the Nepali edition prints it and as here, it is 1.
Answer
Question 9
If g + h + f = 0, prove that .
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Question 10
If r + s + t = 0, prove that .
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Question 11
If xyz = 1, prove that .
Hint
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How the exam asks it
Grade 9’s final exam is set to CDC’s specification grid for Compulsory Mathematics, Grades 9–10. There, Algebra is 15 marks in three questions. In the grid’s Grade 9 model paper, indices are question 9b, beside the HCF in 9a (Unit 10):
| Part | Level | Marks | What it asks |
|---|---|---|---|
| b. | application | 3 | simplify three brackets with letters in the indices |
The three marks go with three steps: one bracket at a time to a single power (division law), the indices multiplied out with an identity (power law), and the sum of the indices with . Write each step, even when you can see the answer is 1. Here is one of our own in that shape.
Mixed exercise
This exercise mixes all of algebra: factorization Grade 9: Unit 9, HCF and LCM Grade 9: Unit 10, linear equations Grade 9: Unit 11 and indices.
Question 1
Factorize .
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Factorize .
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Factorize .
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Factorize .
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Factorize .
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Factorize .
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Factorize .
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Factorize .
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Question 2
Find the HCF of , and .
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Find the HCF of , and .
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Find the HCF of , and .
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Find the HCF of , and .
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Question 3
Find the LCM of , and .
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Find the LCM of , and .
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Find the LCM of , and .
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Find the LCM of a(a + c) − b(b + c), b(a + b) − c(c + a) and c(b + c) − a(a + b).
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Question 4
The HCF and LCM of two algebraic expressions are 2u + 5v and 3u(2u + 5v)(2u − 5v). If one expression is , find the other.
Answer
Question 5
Find the HCF and LCM of , and .
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Find the HCF and LCM of , and .
The textbook prints the third expression as , which has no factor in common with the other two, so the book’s numbers don’t fit its own answer. , as here, does.
Answer
Question 6
Solve by substitution: 2x + 3y = 8, 3x + 2y = 7.
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Solve by substitution: , .
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Solve by substitution: 5x − 2y = 10, 4x − 3y = −6.
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Question 7
Solve by elimination: , .
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Solve by elimination: , .
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Question 8
Muna’s present age is 4 years more than twice Umesh’s. Umesh’s present age is 2 years more than of Muna’s. Find the difference between their ages.
Answer
Question 9
The present ages of Tilak and Bimala are in the ratio 5 : 7. Eight years ago the ratio was 7 : 13. Find their present ages.
Answer
Question 10
In a two-digit number the ones digit is three times the tens digit. If 36 is added to the number, its digits change places. Find the number.
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Question 11
Zeenat is setting up a new office. At a shop, 3 tables and 2 chairs cost Rs 1,900, and 2 tables and 4 chairs cost Rs 1,800. Find the price of a table and of a chair, and the cost of one table and four chairs.
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Question 12
Simplify .
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Question 13
Simplify .
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Question 14
Simplify .
The textbook prints the last bracket as . Then nothing cancels and the answer isn’t a single power (x = 2, y = 1 gives about 439 instead of 512), so the book’s numbers don’t work. With , as here, it is Example 5 again.
Answer
Question 15
If , prove that .
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Practice every question in the book, and keep track of the ones you miss →
Where these facts come from
- Folding paper (the opening, PaperFolds, Back to the paper): Wikipedia, “Britney Gallivan” (January 2002, a 1,200 m strip of toilet paper folded in half 12 times; her formula for the length needed), read November 2026. The Moon’s average distance, 3,84,400 km, and 1 au, 14,96,00,000 km: NASA. Everest, 8,848.86 m: the Nepal–China survey announced in December 2020. The Space Station’s height, about 400 km: NASA. Office paper about 0.1 mm thick (80 g/m² paper).
- The guitar (fact in 12.2): Wikipedia, “Fret” (the twelfth fret divides the string in two halves; successive frets in the ratio of the twelfth root of two, about 1.059463), read November 2026.
- Unit prefixes and the red blood cell: the SI prefixes (BIPM, The International System of Units); Wikipedia, “Red blood cell” (about 6.2–8.2 micrometers across), read November 2026.
- The story: Wikipedia, “Exponentiation” (Chuquet’s notation in the 15th century; “exponent” coined by Stifel in 1544; Descartes, 1637, “aa, or a²”) and “Nicolas Chuquet” (Triparty, 1484; may have been the first to recognize zero and negative numbers as exponents), read November 2026.
- Kepler’s third law (fact in 12.3): Wikipedia, “Kepler’s laws of planetary motion” (published 1619); Jupiter’s orbit, 5.2 au and 11.86 years, NASA’s Jupiter fact sheet.
- The exam’s question (For the exam, How the exam asks it): CDC, Specification Grid, Compulsory Mathematics, Grades 9–10 (Algebra 15 marks in three questions) and its Grade 9 model question 9b (3A); our question is our own.
- Every answer: worked out here and checked with a computer algebra program (SymPy).
A companion to your textbook, not a replacement for it: study from the book your school uses, and where the two differ, follow your teacher. Found a mistake? Please tell us.
