We've updated our
Privacy Policy effective December 15. Please read our updated Privacy Policy and tap

Guide allo studio > Prealgebra

Finding the Reciprocal of a Number

Learning Outcomes

  • Find the reciprocal of a fraction
  • Recognize the difference between absolute value, reciprocal and opposite of a fraction or number

The fractions 23\frac{2}{3} and 32\frac{3}{2} are related to each other in a special way. So are −107-\frac{10}{7} and −710-\frac{7}{10}. Do you see how? Besides looking like upside-down versions of one another, if we were to multiply these pairs of fractions, the product would be 11.

23⋅32=1 and −107(−710)=1\frac{2}{3}\cdot \frac{3}{2}=1\text{ and }-\frac{10}{7}\left(-\frac{7}{10}\right)=1 Such pairs of numbers are called reciprocals.

Reciprocal

The reciprocal of the fraction ab\frac{a}{b} is ba\frac{b}{a}, where a≠0a\ne 0 and b≠0b\ne 0, A number and its reciprocal have a product of 11. ab⋅ba=1\frac{a}{b}\cdot \frac{b}{a}=1
To find the reciprocal of a fraction, we invert the fraction. This means that we place the numerator in the denominator and the denominator in the numerator. To get a positive result when multiplying two numbers, the numbers must have the same sign. So reciprocals must have the same sign. To find the reciprocal, keep the same sign and invert the fraction. The number zero does not have a reciprocal. Why? A number and its reciprocal multiply to 11. Is there any number rr so that 0⋅r=1?0\cdot r=1? No. So, the number 00 does not have a reciprocal.

Example

Find the reciprocal of each number. Then check that the product of each number and its reciprocal is 11.
  1. 49\frac{4}{9}
  2. −16-\frac{1}{6}
  3. −145-\frac{14}{5}
  4. 77
Solution: To find the reciprocals, we keep the sign and invert the fractions.
1.
Find the reciprocal of 49\frac{4}{9} . The reciprocal of 49\frac{4}{9} is 94\frac{9}{4} .
Check:
Multiply the number and its reciprocal. 49⋅94\frac{4}{9}\cdot \frac{9}{4}
Multiply numerators and denominators. 3636\frac{36}{36}
Simplify. 1✓1\quad\checkmark
2.
Find the reciprocal of −16-\frac{1}{6} . −61-\frac{6}{1}
Simplify. −6-6
Check: −16⋅(−6)-\frac{1}{6}\cdot \left(-6\right)
1✓1\quad\checkmark
3.
Find the reciprocal of −145-\frac{14}{5} . −514-\frac{5}{14}
Check: −145⋅(−514)-\frac{14}{5}\cdot \left(-\frac{5}{14}\right)
7070\frac{70}{70}
1✓1\quad\checkmark
4.
Find the reciprocal of 77 .
Write 77 as a fraction. 71\frac{7}{1}
Write the reciprocal of 71\frac{7}{1} . 17\frac{1}{7}
Check: 7⋅(17)7\cdot \left(\frac{1}{7}\right)
1✓1\quad\checkmark

Try It

#141842 [ohm_question height="270"]141842[/ohm_question]
In the following video we will show more examples of how to find the reciprocal of integers, fractions and mixed numbers. https://youtu.be/IM991IqCi44 In a previous chapter, we worked with opposites and absolute values. The table below compares opposites, absolute values, and reciprocals.
Opposite Absolute Value Reciprocal
has opposite sign is never negative has same sign, fraction inverts

Example

Fill in the chart for each fraction in the left column:
Number Opposite Absolute Value Reciprocal
−38-\frac{3}{8}
12\frac{1}{2}
95\frac{9}{5}
−5-5

Answer: Solution: To find the opposite, change the sign. To find the absolute value, leave the positive numbers the same, but take the opposite of the negative numbers. To find the reciprocal, keep the sign the same and invert the fraction.

Number Opposite Absolute Value Reciprocal
−38-\frac{3}{8} 38\frac{3}{8} 38\frac{3}{8} −83-\frac{8}{3}
12\frac{1}{2} −12-\frac{1}{2} 12\frac{1}{2} 22
95\frac{9}{5} −95-\frac{9}{5} 95\frac{9}{5} 59\frac{5}{9}
−5-5 55 55 −15-\frac{1}{5}

Try It

Fill in the chart for each number given:
Number Opposite Absolute Value Reciprocal
−58-\frac{5}{8}
14\frac{1}{4}
83\frac{8}{3}
−8-8

Answer: No alt text

Try it

#146026 [ohm_question height="270"]146026[/ohm_question]
The following video provides more examples of finding the opposite of a number. https://youtu.be/suk9KMzOKkU The next video shows how to find the absolute value of an integer. https://youtu.be/lY5ksjix5Kg

Licenses & Attributions