Powers of rational numbers
A sliotar dropped on a hard pitch rises to $\frac{2}{3}$ of its previous height on every bounce: after 4 bounces it will be at $\left(\frac{2}{3}\right)^4$ of its initial height. And on calculators you will see things like $10^{-3}$. Here you will learn what powers of fractions and negative indices mean (negative indices are Higher Level only).
Powers of a fraction
Raising a fraction to a power means raising the numerator and the denominator to that power:
$$\left(\frac{2}{5}\right)^3 = \frac{2}{5} \times \frac{2}{5} \times \frac{2}{5} = \frac{2^3}{5^3} = \frac{8}{125}$$A square tile with side $\frac{2}{5}$ m has an area of $\left(\frac{2}{5}\right)^2 = \frac{4}{25}$ m²: a square metre holds 25 little squares of side $\frac{1}{5}$ m, and the tile covers 4 of them.
We have a small square tile for a mosaic. The side of the tile measures $\frac{2}{5}$ of a metre. Calculate the area occupied by this tile, expressing the result as a…
We have a small square tile for a mosaic. The side of the tile measures $\frac{2}{5}$ of a metre. Calculate the area occupied by this tile, expressing the result as a fraction.
Use the following visual diagram to help you:
Sometimes we confuse the size of fractions when we raise them to a power. Look at the following graph showing the unit, $\left(\frac{1}{2}\right)^1$ and…
Sometimes we confuse the size of fractions when we raise them to a power. Look at the following graph showing the unit, $\left(\frac{1}{2}\right)^1$ and $\left(\frac{1}{2}\right)^2$.
Based on the logic above, order the following powers from smallest to largest:
$$ \left( \frac{1}{3} \right)^2, \quad \left( \frac{1}{3} \right)^1, \quad \left( \frac{1}{3} \right)^3 $$Negative indices
Look at the table: each time the index goes down by one, the result is divided by the base. If we continue the pattern below 0, fractions appear.
Explore with base 2 or 10 and negative indices: the result gets tiny, but it never reaches 0 and never becomes negative.
Calculate the numerical value of the following powers. Remember to raise both the numerator and the denominator to the given exponent. Express the result as an…
Calculate the numerical value of the following powers. Remember to raise both the numerator and the denominator to the given exponent. Express the result as an irreducible fraction.
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$\left( \frac{2}{3} \right)^3$
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$\left( \frac{1}{5} \right)^2$
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$\left( \frac{3}{2} \right)^4$
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$\left( \frac{10}{7} \right)^0$
In mathematics, we sometimes need to change the sign of the exponent to perform operations. Rewrite the following powers so that they have a positive exponent, inverting…
In mathematics, we sometimes need to change the sign of the exponent to perform operations. Rewrite the following powers so that they have a positive exponent, inverting the base when necessary. You do not need to calculate the final result, just express the transformed power.
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$\left( \frac{4}{5} \right)^{-2}$
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$\left( \frac{1}{3} \right)^{-4}$
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$5^{-3}$
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$\left( -\frac{2}{7} \right)^{-3}$
Calculate the value of the following powers by first converting them into powers with positive exponents. Remember that the result must be an irreducible fraction. a)…
Calculate the value of the following powers by first converting them into powers with positive exponents. Remember that the result must be an irreducible fraction.
- a) $(\frac{3}{5})^{-2}$
- b) $(\frac{-2}{3})^{-3}$
- c) $-( \frac{1}{4} )^{-2}$
- d) $(\frac{10}{3})^{-1}$
The sign of the result
You only need to look at the base and whether the index is even or odd:
| Base | Even index | Odd index |
|---|---|---|
| Positive | $+$ $\quad (2^4 = 16)$ | $+$ $\quad (2^3 = 8)$ |
| Negative | $+$ $\quad ((-2)^4 = 16)$ | $-$ $\quad ((-2)^3 = -8)$ |
With a negative index (Higher Level only) the same applies: $\left(-\frac{1}{2}\right)^{-3} = (-2)^3 = -8$.
Determine the sign (positive or negative) of the result of the following powers without carrying out the full calculation. Justify your answer based on whether the base…
Determine the sign (positive or negative) of the result of the following powers without carrying out the full calculation. Justify your answer based on whether the base is negative and whether the exponent is even or odd.
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$\left( -\frac{1}{2} \right)^4$
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$\left( -\frac{3}{4} \right)^3$
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$\left( \frac{5}{2} \right)^{-2}$
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$\left( -\frac{2}{3} \right)^{-5}$
Working with powers of fractions
All the properties from the previous section still hold with fractional bases. Examples and exercises that use negative indices are Higher Level only.
Calculate the following powers that have the same exponent but different bases. Remember that you can multiply the bases first and raise the result to the common…
Calculate the following powers that have the same exponent but different bases. Remember that you can multiply the bases first and raise the result to the common exponent. Express the result as a simplified fraction.
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$\left( \frac{2}{5} \right)^3 \times \left( \frac{5}{4} \right)^3$
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$\left( \frac{1}{3} \right)^2 \times 6^2$
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$\left( \frac{3}{2} \right)^4 : \left( \frac{1}{2} \right)^4$
The key idea for powers with the same base is below:
Finding an unknown index
Some of the examples and exercises here use negative indices, which are Higher Level only.
If two powers with the same base are equal, their indices are equal. So to solve an equation like $\left(\frac{2}{3}\right)^x = \frac{8}{27}$, write both sides as powers of the same base and compare the indices.
Use the properties of powers to simplify the following expressions. Leave your answer expressed as a single power (you do not need to calculate the final value). 1.…
Use the properties of powers to simplify the following expressions. Leave your answer expressed as a single power (you do not need to calculate the final value).
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$\left( \frac{3}{4} \right)^5 \times \left( \frac{3}{4} \right)^2$
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$\left( \frac{2}{5} \right)^7 : \left( \frac{2}{5} \right)^4$
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$\left( \frac{1}{2} \right)^3 \times \left( \frac{1}{2} \right)$
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$\left[ \left( \frac{5}{3} \right)^2 \right]^4$
Find the missing number in each box to make the equations true. Write the complete equation in your notebook. 1. $\left( \frac{2}{3} \right)^{\square} = \frac{8}{27}$ 2.…
Find the missing number in each box to make the equations true. Write the complete equation in your notebook.
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$\left( \frac{2}{3} \right)^{\square} = \frac{8}{27}$
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$\left( \frac{5}{2} \right)^{\square} = 1$
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$\left( \frac{1}{4} \right)^{-2} = \square$
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$\left( \frac{3}{7} \right)^3 \times \left( \frac{3}{7} \right)^{\square} = \left( \frac{3}{7} \right)^5$
Find the value of $x$ in each of the following equations to make them true. Think about the properties of powers with the same base. a) $(\frac{1}{2})^x = 32$ b)…
Find the value of $x$ in each of the following equations to make them true. Think about the properties of powers with the same base.
- a) $(\frac{1}{2})^x = 32$
- b) $(\frac{3}{5})^4 \times (\frac{3}{5})^x = (\frac{3}{5})^{-1}$
- c) $[(\frac{2}{9})^x]^2 = (\frac{2}{9})^{10}$
Back to the start: After 4 bounces, the sliotar reaches $\left(\frac{2}{3}\right)^4 = \frac{2^4}{3^4} = \frac{16}{81}$ of its initial height (less than 20%), and on the calculator screen $10^{-3} = \frac{1}{10^3} = \frac{1}{1000} = 0.001$.