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Thought-Process to Discover Knowledge

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mathsFractionsArithmetics with Fractions

Addition and Subtraction of Fractions

Addition or Subtraction of fractions having different place values or denominator is explained. The following are explained for fractions.

 •  addition in first principles -- combining two quantities and measuring the combined

 •  subtraction in first principles -- taking away a part of a quantity and measuring the remaining quantity.

 •  Simplified procedure : Sign property of Addition and converting to like fractions

 •  Simplified procedure : Sign property of Subtraction and Converting to like fractions



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In whole numbers, we had studied the following.

Addition - First Principles : Two numbers are considered, each of which represents a count or measurement. The quantities represented by the numbers are combined to form a result representing a combined count or measurement. The combined count or measurement is the result of addition.

eg: `20` and `13` are combined together `20+13 = 33`.

`20` is an addend

`13` is also an addend

`33` is the sum

Subtraction - First Principles : Two numbers are considered, each of which represents a count or measurement. From one amount represented by the first number, the amount represented by the second is taken away to form a result representing the remaining amount. The count or measurement of the remaining amount is the result of subtraction.

eg: `20-13 = 7`

`20` is the minuend

`13` is the subtrahend

`7` is the difference

Given two fractions in two colors in the figure. What is the sum of these?addition

  • sum is not `3+2` as the parts are in different sizes
  • sum is not `3+2` as the parts are in different sizes
  • the sum is `5` pieces

The answer is 'sum is not `3+2` as the parts are in different sizes'. This means that the place values are different.

Given two fractions `3/8` and `2/8` in the figure, what is the sum?addition like fractions

  • sum is not `3+2` as the parts are in different sizes
  • the sum is `5/8`
  • the sum is `5/8`

The answer is 'the sum is `5/8`'. The fractions have identical place values and so the numerators are added.

Given two fractions `1/4` and `3/8` in the figure, what is the sum?addition unlike like fractions

  • addition is not possible as the parts are different
  • convert them to like fractions to add
  • convert them to like fractions to add

The answer is 'converting them to like fractions to add'.

Given two fractions `1/4` and `3/8` in the figure, the fraction `1/4` is converted to equivalent fraction `2/8`. What is the sum of `1/4` and `3/8`?addition unlike like fractions

  • addition is not possible
  • the sum is `5/8`
  • the sum is `5/8`

The answer is 'sum is `5/8`'.

The sum is calculated as `2/8 + 3/8 = 5/8`

Given two fractions `3/8` and `2/8` in the figure, what is the difference `3/8 - 2/8`?subtraction like fractions

  • difference is not `3-2` as the parts are in different sizes
  • the difference is `1/8`
  • the difference is `1/8`

The answer is 'the difference is `1/8`'. The fractions have identical place values and so the numerators are subtracted.

Given two fractions `3/8` and `1/4` in the figure, what is the difference `3/8 - 1/4`?subtraction unlike like fractions

  • subtraction is not possible
  • convert them to like fractions to subtract
  • convert them to like fractions to subtract

The answer is 'converting them to like fractions to subtract'.

Given two fractions `3/8` and `1/4` in the figure, the fraction `1/4` is converted to equivalent fraction `2/8`. What is the difference `3/8-1/4`?subtraction unlike like fractions

  • subtraction is not possible
  • the difference is `1/8`
  • the difference is `1/8`

The answer is 'difference is `1/8`'.

The difference is calculated as `3/8 - 2/8 = 1/8`

Convert the fractions to "like fractions" (having same place value) to add or subtract numerators.

Fractions are directed numbers with positive and negative values.

So far we considered only positive fractions while learning addition and subtraction. Now, let us consider addition and subtraction of positive and negative fractions.

Integer Addition -- First Principles : Directed whole numbers addition is combining the two amounts with direction information taken into account.

Examples are:
 •  `text(received:)3` + `text(received:)2` = `text(received:)5`
`3+2 = 5`

 •  `text(received:)3` + `text(given:)2` = `text(received:)1`
`3+(−2) = 3−2 = 1`

 • `text(given:)3` & `text(received:)2` = `text(given:)1`
`-3+2 = -1`

 • `text(received:)3` & `text(given:)5` = `text(given:)2`
`3+(−5) = 3−5 = −2`

 • `text(given:)3` + `text(given:)2` = `text(given:)5`
`−3+(−2) = −5`

What is the sum of `-1/4` and `1/2`?

  • `3/4`
  • `1/4`
  • `1/4`

The answer is "`1/4`".

The given fractions are unlike fractions and so are converted to like fractions.

`-1/4 = text(given:)1/4`

`1/2 = text(received:)2/4` (like fraction)

Given 1 count and received 2 counts together is received 1 count.

So, `-1/4 + 1/2 = + 1/4`

What is the sum of `1/4` and `(-1)/2`?

  • `+1/4`
  • `(-1)/4`
  • `(-1)/4`

The answer is "`-1/4`".

The given fractions are unlike fractions and so are converted to like fractions.

`1/4 = text(received:)1/4`

`-1/2 = text(given:)2/4`

Received `1` count and given `2` counts together is given `1` count.

So, `1/4 + (-1)/2 = (-1)/4`

What is the sum of `(-1)/4` and `(-1)/2`?

  • `-3/4`
  • `-3/4`
  • `-1/4`

The answer is "`-3/4`".

The given fractions are unlike fractions and so are converted to like fractions.

`(-1)/4 = text(given:)1/4`

`(-1)/2 = text(given:)2/4`

Given `1` count and given `2` counts together is given `3` counts.

So, `-1/4 + (-1)/2 = (-3)/4`

Integer Subtraction -- First Principles : Directed whole numbers subtraction is taking away an amount from another with direction information taken into account.

Examples are :
 •  from `text(received:)5`, taking away `text(received:)2` is equivalently, combining `text(received:)5` and `text(given:)2`;
`5-2 = 5+(-2)`
 •  from `text(received:)5`, taking away `text(given:)2` is equivalently, combining `text(received:)5` and `text(received:)2`;
`5- (-2) = 5+(+2)`
 •  from `text(given:)5`, taking away `text(received:)2` is equivalently, combining `text(given:)5` and `text(given:)2`;
`(-5)- 2 = (-5)+(-2)`
 •  from `text(given:)5`, taking away `text(given:)2` is equivalently, combining `text(given:)5` and `text(received:)2`;
`(-5)- (-2) = (-5)+(+2)`

What is the difference of `-1/4 - 1/2`?

  • `-3/4`
  • `-3/4`
  • `1/4`

The answer is "`1/4`".

The given fractions are unlike fractions and so are converted to like fractions.

So `(-1)/4 - 1/2 `

`= (-1)/4 + (-2)/4`

`= (-3)/4`

Addition of Fractions: First Principles Addition is performed only when addends are like fractions.

Any unlike fractions are converted to like fractions to perform addition.

Fractions are directed numbers, having positive and negative values. The direction information is taken into account while performing addition.

Subtraction of Fractions : first principles Subtraction by subtrahend is addition of additive inverse of subtrahend.

We learned the addition and subtraction of fractions in first principles. In whole numbers and integers, we studied simplified procedures. Let us review those procedures and develop a simplified procedure for addition and subtraction of fractions.

In whole numbers, we had studied the following.

Addition by Place-value with regrouping -- simplified procedure : Two numbers are added as per the following procedure:

 •  the place-value positions are arranged units under units, `10`s under `10`s, etc.

 •  the units are added and if the result has `10` numbers, then it is carried to the `10`s place.

 •  The addition is continued to the higher place-value position. addition simplified procedure The carry over is the simplification of combining `10` of a place value to a higher place value.

Addition of Integers Simplified Procedure : Two numbers with signs positive or negative are added as follows.

sign-property of addition

 •  +ve `+` +ve is whole number addition

 •  +ve `+` -ve is subtraction as given below
     absolute values of the two numbers are compared
     subtract the number of smaller absolute value from the number of larger absolute value. The difference is the absolute value of the result.
     the sign of result is the sign of the number with larger absolute value

 •  -ve `+` -ve is addition of absolute values with negative sign.

The addition or subtraction of absolute values in the procedure are detailed in addition by place-value and subtraction by place-value

Integer Subtraction Simplified Procedure : Sign-Property of Integer Subtraction:

Integer subtraction is handled as integer addition with addends (a) minuend and (b) the negative of subtrahend.

`text(minuend )-text( subtrahend )`
` = text( minuend as addend )``+ text( negative of subtrahend as addend)`

Convert Unlike to Like fractions The procedural simplification is as follows.
Two fractions `p/q` and `l/m` are given. Find the LCM of denominators `q` and `m` such that `text(LCM) = qxxi = mxxj`. Then convert the fractions to equivalent fractions `(p xx i)/(q xx i)` and `(l xx j)/(m xx j)`. These are like fractions.

Addition or Subtraction of Fractions: Simplified Procedure Convert the unlike fractions to like fraction by the procedure.

Addition and subtraction of numerator uses the following from whole numbers and integers

 •  Sign Property of Integer addition

 •  Subtraction is Addition of Additive Inverse

 •  Whole numbers Addition by Place value

Solved Exercise Problem:

Find the sum of `323/999` and `297/999`.

  • `620/999`
  • `620/999`
  • `21110/999`

The answer is "`620/999`".

This problem uses the following procedures
Whole number Addition by Place Value

Solved Exercise Problem:

Find the sum of `-3/8` and `2/6`.

  • `-1/24`
  • `-1/24`
  • `17/24`

The answer is "`-1/24`".

This problem uses the following procedures
conversion into like fractions
sign property of integer addition

Solved Exercise Problem:

Find the sum of `1` and `-1/4`

  • `-1 1/4`
  • `3/4`
  • `3/4`

The answer is "`3/4`". To solve this problem, the fractions are converted to like fractions and signed property of integer addition is used.

Solved Exercise Problem:

Find the difference of `-3/8 - 2/6`.

  • `-1/24`
  • `-17/24`
  • `-17/24`

The answer is "`-17/24`"

This problem uses the following procedures
conversion into like fractions
conversion of subtraction into addition of negative of subtrahend
sign property of integer addition

Solved Exercise Problem:

Find the sum of `1- (-1)/4`

  • `1 1/4`
  • `3/4`
  • `3/4`

The answer is "`1 1/4`"

                            
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