MYP 3 · Maths
NUMBER
Index notation
QUESTION 1 [3 marks] — Criterion A
Medium
Simplify each expression, showing full working:
a.
$2^3 \times 2^4$
[1] Show Solution
$$2^3\times2^4 = 2^{3+4} = 2^7 = 128$$
b.
$\dfrac{5^6}{5^2}$
[1] Show Solution
$$5^{6-2} = 5^4 = 625$$
c.
$(3^2)^3$
[1] Show Solution
$$3^{2\times3} = 3^6 = 729$$
QUESTION 2 [4 marks] — Criterion A
Medium
A single bacterium in a petri dish doubles in number every hour.
a.
Write an expression, using index notation, for the number of bacteria after $n$ hours.
[1] Show Solution
$$2^n$$
b.
Find the number of bacteria after 8 hours.
[1] Show Solution
$$2^8 = 256$$
c.
Find how many more hours it takes to go from 256 bacteria to over 10000 bacteria.
[2] Show Solution
$2^{13}=8192$ and $2^{14}=16384$, so it takes until hour 14 to exceed 10000 — that's $14-8=6$ more hours.
QUESTION 3 [5 marks] — Criterion B
Medium
Investigate the pattern in the last digit of increasing powers of 2.
a.
List the last digit of $2^1, 2^2, 2^3, \ldots, 2^8$.
[2] Show Solution
$2,4,8,6,2,4,8,6$ (for powers 1 through 8).
b.
Describe the pattern you observe.
[1] Show Solution
The last digits repeat in a cycle of 4: $2,4,8,6$, then repeat.
c.
Using the pattern, predict the last digit of $2^{23}$ without calculating the full value.
[2] Show Solution
$23 \div 4 = 5$ remainder $3$, so $2^{23}$ has the same last digit as $2^3$, which is $8$.
QUESTION 4 [5 marks] — Criterion C
Medium
A student simplifies $2^3 \times 3^2$ as $6^5$.
a.
Evaluate both $2^3\times3^2$ and $6^5$ directly to show whether the student's simplification is correct.
[2] Show Solution
$2^3\times3^2 = 8\times9=72$. $6^5=7776$. These are not equal, so the student's simplification is incorrect.
b.
Explain, using the index laws, why the rule $a^m \times b^n = (ab)^{m+n}$ that the student seems to be using is not valid, and state when index addition rules DO apply.
[3] Show Solution
The index law $a^m\times a^n=a^{m+n}$ only applies when the BASE is the same on both terms. Here the bases (2 and 3) are different, so the exponents cannot simply be combined — each power must be evaluated separately before multiplying.
QUESTION 5 [6 marks] — Criterion D
Medium
A savings amount doubles every 3 years due to compound growth. An initial deposit of \$500 is made.
a.
Write an expression for the amount after $t$ periods of 3 years, and find the amount after 4 such periods (12 years).
[3] Show Solution
Amount $=500\times2^t$. After 4 periods: $500\times2^4=500\times16=8000$.
b.
The saver wants to reach \$50000. Find the smallest whole number of 3-year periods needed, and state the total number of years this takes.
[3] Show Solution
$500\times2^t \ge 50000 \Rightarrow 2^t\ge100$. Since $2^6=64$ and $2^7=128$, $t=7$ periods are needed, i.e. $7\times3=21$ years.
QUESTION 6 [7 marks] — Criterion A
Hard
A single-cell organism divides into 2 every hour. A colony starts with 3 organisms.
a.
Write an expression, using index notation, for the population after $h$ hours, and find the population after 10 hours.
[3] Show Solution
$P(h)=3\times2^h$. After 10 hours: $3\times2^{10}=3\times1024=3072$.
b.
A rival colony starts with just 1 organism, but divides into 3 every hour instead of 2. Find after how many WHOLE hours the rival colony's population first EXCEEDS the original colony's population (assuming both start at $h=0$).
[4] Show Solution
Original: $3\times2^h$. Rival: $1\times3^h=3^h$. Testing values: $h=3$: original$=24$, rival$=27$ (rival exceeds!). $h=2$: original$=12$, rival$=9$ (rival still behind). So the rival colony first exceeds the original after 3 whole hours.
QUESTION 7 [8 marks] — Criterion B
Hard
Investigate the units digit of increasing powers of 7, i.e. $7^1, 7^2, 7^3, \ldots$
a.
Find the units digit of $7^1$ through $7^8$.
[3] Show Solution
$7^1=7$(7), $7^2=49$(9), $7^3=343$(3), $7^4=2401$(1), $7^5=16807$(7), $7^6=\ldots$(9), $7^7=\ldots$(3), $7^8=\ldots$(1). Units digits: $7,9,3,1,7,9,3,1$.
b.
State the repeating cycle length, and predict the units digit of $7^{35}$ using the cycle, showing your reasoning.
[3] Show Solution
The cycle length is 4 ($7,9,3,1$ repeating). $35\div4=8$ remainder $3$, so $7^{35}$ has the same units digit as $7^3$, which is $\boxed{3}$.
c.
Prove that $7^{35}$ is NOT a multiple of 10, using your units-digit finding, without calculating the full value of $7^{35}$.
[2] Show Solution
A number is a multiple of 10 only if its units digit is 0. Since $7^{35}$ has units digit 3 (never 0, as the full 4-digit cycle $7,9,3,1$ never includes 0), $7^{35}$ can never be a multiple of 10.
QUESTION 8 [5 marks] — Criterion C
Hard
A student writes $2^{10}$ as '$2\times10=20$', confusing index notation with multiplication notation.
a.
Precisely define what $2^{10}$ means (using repeated multiplication notation), and calculate its correct value.
[3] Show Solution
$2^{10}$ means $2$ multiplied by itself $10$ times: $\underbrace{2\times2\times2\times\cdots\times2}_{10\text{ factors}} = 1024$, which is very different from $2\times10=20$.
b.
Explain, using the relative SIZES of $2^{10}=1024$ versus $2\times10=20$, why confusing these two notations could lead to serious errors in a real scientific or financial calculation.
[2] Show Solution
The two values differ by a factor of over 50 ($1024$ vs $20$) — in a real context like calculating compound growth, data storage (e.g. computer memory, where $2^{10}$ 'kilobytes' is a standard unit), or population growth, this confusion could lead to a drastically wrong final answer, potentially by orders of magnitude.
QUESTION 9 [6 marks] — Criterion D
Hard
A hard drive's storage capacity doubles with each new model generation. The Generation 1 drive holds $2^8$ MB. Each new generation adds exactly 1 to the exponent.
a.
Find the storage capacity, in MB, of the Generation 5 drive.
[2] Show Solution
Generation 5 exponent $=8+4=12$. Capacity $=2^{12}=4096$ MB.
b.
A consumer wants a drive with AT LEAST 1,000,000 MB (1 TB) of storage. Find the generation number required, and comment on whether 'doubling every generation forever' is a realistic assumption for real hard drive technology over many decades.
[4] Show Solution
Need $2^{8+n}\ge1000000$. Testing: $2^{20}=1048576\ge1000000$ ? (generation where exponent$=20$, i.e. Generation $20-8+1=13$). This 'doubling' pattern (similar to Moore's Law in computing) has historically held reasonably well over shorter timeframes, but is not realistic to extrapolate indefinitely — physical and economic limits eventually slow or halt this kind of exponential growth in real technology.
QUESTION 10 [7 marks] — Criterion A
Hard
A single-cell organism divides into 2 every hour. A colony starts with 3 organisms.
a.
Write an expression, using index notation, for the population after $h$ hours, and find the population after 10 hours.
[3] Show Solution
$P(h)=3\times2^h$. After 10 hours: $3\times2^{10}=3\times1024=3072$.
b.
A rival colony starts with just 1 organism, but divides into 3 every hour instead of 2. Find after how many WHOLE hours the rival colony's population first EXCEEDS the original colony's population (assuming both start at $h=0$).
[4] Show Solution
Original: $3\times2^h$. Rival: $1\times3^h=3^h$. Testing values: $h=3$: original$=24$, rival$=27$ (rival exceeds!). $h=2$: original$=12$, rival$=9$ (rival still behind). So the rival colony first exceeds the original after 3 whole hours.
QUESTION 11 [8 marks] — Criterion B
Hard
Investigate the units digit of increasing powers of 7, i.e. $7^1, 7^2, 7^3, \ldots$
a.
Find the units digit of $7^1$ through $7^8$.
[3] Show Solution
$7^1=7$(7), $7^2=49$(9), $7^3=343$(3), $7^4=2401$(1), $7^5=16807$(7), $7^6=\ldots$(9), $7^7=\ldots$(3), $7^8=\ldots$(1). Units digits: $7,9,3,1,7,9,3,1$.
b.
State the repeating cycle length, and predict the units digit of $7^{35}$ using the cycle, showing your reasoning.
[3] Show Solution
The cycle length is 4 ($7,9,3,1$ repeating). $35\div4=8$ remainder $3$, so $7^{35}$ has the same units digit as $7^3$, which is $\boxed{3}$.
c.
Prove that $7^{35}$ is NOT a multiple of 10, using your units-digit finding, without calculating the full value of $7^{35}$.
[2] Show Solution
A number is a multiple of 10 only if its units digit is 0. Since $7^{35}$ has units digit 3 (never 0, as the full 4-digit cycle $7,9,3,1$ never includes 0), $7^{35}$ can never be a multiple of 10.
QUESTION 12 [5 marks] — Criterion C
Hard
A student writes $2^{10}$ as '$2\times10=20$', confusing index notation with multiplication notation.
a.
Precisely define what $2^{10}$ means (using repeated multiplication notation), and calculate its correct value.
[3] Show Solution
$2^{10}$ means $2$ multiplied by itself $10$ times: $\underbrace{2\times2\times2\times\cdots\times2}_{10\text{ factors}} = 1024$, which is very different from $2\times10=20$.
b.
Explain, using the relative SIZES of $2^{10}=1024$ versus $2\times10=20$, why confusing these two notations could lead to serious errors in a real scientific or financial calculation.
[2] Show Solution
The two values differ by a factor of over 50 ($1024$ vs $20$) — in a real context like calculating compound growth, data storage (e.g. computer memory, where $2^{10}$ 'kilobytes' is a standard unit), or population growth, this confusion could lead to a drastically wrong final answer, potentially by orders of magnitude.
QUESTION 13 [6 marks] — Criterion D
Hard
A hard drive's storage capacity doubles with each new model generation. The Generation 1 drive holds $2^8$ MB. Each new generation adds exactly 1 to the exponent.
a.
Find the storage capacity, in MB, of the Generation 5 drive.
[2] Show Solution
Generation 5 exponent $=8+4=12$. Capacity $=2^{12}=4096$ MB.
b.
A consumer wants a drive with AT LEAST 1,000,000 MB (1 TB) of storage. Find the generation number required, and comment on whether 'doubling every generation forever' is a realistic assumption for real hard drive technology over many decades.
[4] Show Solution
Need $2^{8+n}\ge1000000$. Testing: $2^{20}=1048576\ge1000000$ ? (generation where exponent$=20$, i.e. Generation $20-8+1=13$). This 'doubling' pattern (similar to Moore's Law in computing) has historically held reasonably well over shorter timeframes, but is not realistic to extrapolate indefinitely — physical and economic limits eventually slow or halt this kind of exponential growth in real technology.