Find the Reciprocal

Reciprocals are helpful in all sorts of algebraic equations. For example when you are dividing one fraction by another you multiply the first by the Reciprocal of the 2nd. You might also need reciprocals when finding equations of lines.

Steps

Finding the Reciprocal of a Fraction or Whole Number

  1. Find the reciprocal of a fraction by flipping it. The definition of "reciprocal" is simple. To find the reciprocal of any number, just calculate "1 ÷ (that number)." For a fraction, the reciprocal is just a different fraction, with the numbers "flipped" upside down (inverted).[1]
    • For instance, the reciprocal of 3/4 is 4/3.
  2. Write the reciprocal of a whole number as a fraction. Again, the reciprocal of a number is always 1 ÷ (that number). For a whole number, write that as a fraction; there's no point in calculating it out to a decimal.
    • For instance, the reciprocal of 2 is 1 ÷ 2 = 1/2.

Finding the Reciprocal of a Mixed Number

  1. Identify a mixed number. Mixed numbers are part whole number and part fraction, such as 24/5. There are two steps to finding the reciprocal of a mixed number, explained below.
  2. Change-Mixed-Numbers-to-Improper-Fractions. Remember, the number 1 can always be written as (number)/(the same number), and fractions with the same denominator (lower number) can be added together. Here's an example with 24/5:
    • 24/5
    • = 1 + 1 + 4/5
    • = 5/5 + 5/5 + 4/5
    • = (5+5+4)/5
    • = 14/5.
  3. Flip the fraction. Once the number is written entirely as a fraction, you can find the reciprocal just like you would with any fraction: by flipping it.
    • In the example above, the reciprocal of 14/5 is 5/14.

Finding the Reciprocal of a Decimal

  1. Change it to a fraction if possible. You might recognize some common decimal numbers that can easily be Convert-a-Decimal-to-a-Fraction. For instance, 0.5 = 1/2, and 0.25 = 1/4. Once in fraction form, just flip the fraction to find the reciprocal.
    • For instance, the reciprocal of 0.5 is 2/1 = 2.
  2. Write out a division problem. If you can't change it to a fraction, calculate the reciprocal of that number as a division problem: 1 ÷ (the decimal). You can use a calculator to solve this, or continue on to the next step to solve it by hand.
    • For example, you can find the reciprocal of 0.4 by calculating 1 ÷ 0.4.
  3. Change the division problem to use whole numbers. The first step to Divide-Decimals is to move the decimal point until all the numbers involved are whole numbers. As long as you move the decimal point the same number of spaces for both numbers, you'll get the correct answer.
    • For example, you can take 1 ÷ 0.4 and rewrite it as 10 ÷ 4. In this case, you've moved each decimal place one space to the right, which is the same as multiplying each number by ten.
  4. Solve the problem using long division. Use long division techniques to calculate the reciprocal. If you calculate it for 10 ÷ 4, you'll get the answer 2.5, the reciprocal of 0.4.

Tips

  • A numbers negative reciprocal is the same as the regular reciprocal, multiplied by negative one.[2] For instance, the negative reciprocal of 3/4 is -4/3.
  • The reciprocal is sometimes called the "multiplicative inverse."[3]
  • The number 1 is its own reciprocal, since 1 ÷ 1 = 1.
  • The number 0 does not have a reciprocal, since 1 ÷ 0 is undefined.[1]

Related Articles

Sources and Citations

  1. ↑ 1.0 1.1 http://www.mathsisfun.com/reciprocal.html
  2. ↑ http://www.mathwarehouse.com/negative-reciprocals/
  3. ↑ http://www.cliffsnotes.com/cliffsnotes/math/in-math-what-does--i-reciprocal-i-mean

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