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Secondary 3 · Mathematics
Powers and roots · Powers

Powers of Rational Numbers

A ball bounces back up to $\frac{2}{3}$ of its previous height on each bounce: after 4 bounces it will be at $\left(\frac{2}{3}\right)^4$ of its starting height. And on calculators you'll see things like $10^{-3}$. Here you'll learn what powers of fractions mean, and what negative exponents are.

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 floor tile with side length $\frac{2}{5}$ m has an area of $\left(\frac{2}{5}\right)^2 = \frac{4}{25}$ m²: a square meter fits 25 little squares with side length $\frac{1}{5}$ m, and the tile covers 4 of them.

1 m 2/5 (2/5)² = 4/25 m² 4 little squares out of 25 When you raise a fraction less than 1 to a power, the result gets smaller.
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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.

  1. $\left( \frac{2}{3} \right)^3$

  2. $\left( \frac{1}{5} \right)^2$

  3. $\left( \frac{3}{2} \right)^4$

  4. $\left( \frac{10}{7} \right)^0$

Solve
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We have a small square tile for a mosaic. The side of the tile measures $\frac{2}{5}$ of a meter. 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 meter. Calculate the area occupied by this tile, expressing the result as a fraction.

Use the following visual diagram to help you:

1/25 2/5 m 2/5 m 1 m (5/5) 1 m Area of the tile: A = l² A = (2/5)² A = ?
Solve
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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 $\left(\frac{1}{2}\right)^2$.

> > 1 1 Unit (1/2)¹ 1/2 Half (1/2)² 1/4 Quarter

Based on the logic above, order the following powers from smallest to largest:

$$ \left( \frac{1}{3} \right)^2, \quad \left( \frac{1}{3} \right)^1, \quad \left( \frac{1}{3} \right)^3 $$
Solve

Negative Exponents

Look at the table: each time the exponent goes down by one, the result is divided by the base. If we continue the pattern below 0, fractions appear.

2³2²2¹2⁰2⁻¹2⁻² 84211/21/4 ÷ 2 at each step

Explore with base 2 or 10 and negative exponents: the result gets tinier and tinier, but it never reaches 0 and never becomes negative.

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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.

  1. $\left( \frac{4}{5} \right)^{-2}$

  2. $\left( \frac{1}{3} \right)^{-4}$

  3. $5^{-3}$

  4. $\left( -\frac{2}{7} \right)^{-3}$

Solve
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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.

  1. a) $(\frac{3}{5})^{-2}$
  2. b) $(\frac{-2}{3})^{-3}$
  3. c) $-( \frac{1}{4} )^{-2}$
  4. d) $(\frac{10}{3})^{-1}$
Solve

The Sign of the Result

You only need to look at the base and whether the exponent is even or odd:

Base Even exponent Odd exponent
Positive $+$ $\quad (2^4 = 16)$ $+$ $\quad (2^3 = 8)$
Negative $+$ $\quad ((-2)^4 = 16)$ $-$ $\quad ((-2)^3 = -8)$

With a negative exponent the same rule applies: $\left(-\frac{1}{2}\right)^{-3} = (-2)^3 = -8$.

(−) × (−) = + (−) × (−) × (−) = − Minus signs "cancel out" in pairs: if one is left over, the result is negative.
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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.

  1. $\left( -\frac{1}{2} \right)^4$

  2. $\left( -\frac{3}{4} \right)^3$

  3. $\left( \frac{5}{2} \right)^{-2}$

  4. $\left( -\frac{2}{3} \right)^{-5}$

Solve

Operating with Powers of Fractions

All the laws of exponents from the previous section still hold with fractional bases and negative exponents:

(3/2)−1 flip (2/3)1 First, all the powers with the SAME base; then, add and subtract exponents.
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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).

  1. $\left( \frac{3}{4} \right)^5 \times \left( \frac{3}{4} \right)^2$

  2. $\left( \frac{2}{5} \right)^7 : \left( \frac{2}{5} \right)^4$

  3. $\left( \frac{1}{2} \right)^3 \times \left( \frac{1}{2} \right)$

  4. $\left[ \left( \frac{5}{3} \right)^2 \right]^4$

Solve
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Use the properties of powers to simplify the following expressions and write the result as a single power with a rational base. You do not need to calculate the final…

Use the properties of powers to simplify the following expressions and write the result as a single power with a rational base. You do not need to calculate the final value, only the simplified power.

  1. a) $(\frac{2}{7})^3 \times (\frac{2}{7})^5 : (\frac{2}{7})^{-2}$
  2. b) $[(\frac{1}{5})^2]^{-3} \times (\frac{1}{5})^4$
  3. c) $(\frac{2}{3})^4 \times (\frac{6}{5})^4$
Solve
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We have a special rubber ball. Each time it bounces off the ground, it reaches a height that is $\frac{2}{3}$ of the previous height. If we drop the ball from a height…

We have a special rubber ball. Each time it bounces off the ground, it reaches a height that is $\frac{2}{3}$ of the previous height.

If we drop the ball from a height of 2 meters:

  1. a) Express as a power the height it reaches after the fourth bounce.
  2. b) Is that height greater or less than 0.3 meters? Justify your answer using the power calculation.
0.3m 2m 1m 0m h₀ h₁ h₂ h₃ h₄ = ? × 2/3 × 2/3 × 2/3 × 2/3
Solve

Summary

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