Negative power
What does this number mean ? Which order of 1, 2, 3 and 4 makes the
highest value ? Which makes the lowest ?
Problem
Image
What does this number mean ?
Which order of $1, 2, 3$ and $4$ makes the highest value ?
Which makes the lowest ?
Getting Started
- What does $4$ to the power $3$ mean ?
- What does $4$ to the power $-3$ mean ?
- What does $-4$ to the power $3$ mean ?
- What does $-4$ to the power $-3$ mean ?
Student Solutions
We had a number of good answers, with commentary, on this problem including from Samantha (The Steele School), and Helen (Lady Margaret School).
Any calculation like this is the same as the bottom number to the power of all the other numbers multiplied together.
So all I needed to try out was :
$-4 \text{ to the power of }(-3 \times-2 \times-1)$ which is $-(4 \text{ to the power of } -6)$
$-3 \text{ to the power of } (-4 \times-2 \times-1)$ which is $-(3 \text{ to the power of } -8)$
$-2 \text{ to the power of } (-3 \times-4 \times-1)$ which is $-(2 \text{ to the power of } -12) $
$-1 \text{ to the power of } (-2 \times-3 \times-4)$ which is $-(1 \text{ to the power of } -24) $
That's each number as the bottom and the rest making a product to be the power or index number.
I noticed that having $4$ or $2$ at the bottom gives the same end result : $4$ to the power of $-6$ is the same as $2$ to the power of $-12$, which made me think a bit. I think it happens because $4$ is $2$ squared, and having $4$ at the bottom is like having only $2$ at the bottom and an extra factor of $2$ in the product that makes the power. But swapping the $2$ at the bottom and putting the $4$ in the power stack also makes the product bigger by a factor of $2$.
So my four calculations, left as fractions, came out like this :
$$1/4096, 1/6561, 1/4096, 1$$
So $1$ is the largest result (putting $1$ at the bottom) and $1/6561$ is the smallest result (putting $3$ at the bottom).
Thank-you to everyone who sent in their results and ideas.
Teachers' Resources
This problem is a good one with which to illustrate the precision of meaning that mathematics often requires - in this case the precedence of the operations.
Superficially the task provides some practice at interpreting negative indices but more deeply it invites students to find a simplification for the 'stacked' indices.
The real variable each time is the value, of the four, which is selected as base. To explore that students might go on to consider which sets of four values will produce the same 'stacked' value, irrespective of stacking order.