Algebra
UK Secondary (7-11)
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Add and subtract algebraic fractions
Lesson

We looked at how to add and subtract fractions in More and Less Parts.

The most important things to remember when adding and subtracting fractions (of any kind) are 

  • we need like denominators (bases)
  • we need to keep our fractions equivalent

Now we are going to build on this knowledge and look at how to add and subtract algebraic fractions.

Worked example

Example 1 

$\frac{4m}{5}-\frac{2}{5}$4m525

Think: The first thing we need to do is check that the denominators are the same.  In this case, both denominators are $5$5, so then we move on to the next step. 

Do: Because our denominators are the same, we can write the numerators together as a single expression over the common denominator. 

$\frac{4m-2}{5}$4m25

Reflect: Because we cannot simplify the numerator $4m-2$4m2 any further, this means this is a simplified as we can get. 

 

Example 2

Let's look at a very similar example, but where the denominators are not initially the same.

$\frac{2y}{3}+\frac{5}{6}$2y3+56

Think: We cannot add or subtract fractions unless the denominators are common.  In this case we one denominator of $3$3, and the other with $6$6.  We need to find a common denominator.  Let's choose $6$6, as it is a common multiple of both $3$3 and $6$6.  

Do:  Change the first fraction to have a denominator of $6$6

$\frac{2y}{3}$2y3 $=$=

$\frac{2y\times2}{3\times2}$2y×23×2

  $=$= $\frac{4y}{6}$4y6

So this means our expression now becomes:

$\frac{2y}{3}+\frac{5}{6}=\frac{4y}{6}+\frac{5}{6}$2y3+56=4y6+56

Success, now we can add the fractions as the denominators are common. 

Do: Now write the numerators as a single expression above the common denominator.

$\frac{4y}{6}+\frac{5}{6}=\frac{4y+5}{6}$4y6+56=4y+56

Reflect:  Is this simplified enough?  As the terms $4y$4y and $5$5 are not like terms, yes, this is simplified as much as we can. 

 

Example 3

Our final example is where we have different denominators, and some simplification to perform at the final step. 

$\frac{3x}{4}+\frac{3x}{2}$3x4+3x2

Think: Our first goal is to have common denominators.  Looking at the denominators we have, $4$4 and $2$2, we can see that $4$4 is a common multiple.  So use $4$4

Do:

$\frac{3x}{4}+\frac{3x}{2}$3x4+3x2 $=$= $\frac{3x}{4}+\frac{3x\times2}{2\times2}$3x4+3x×22×2
  $=$= $\frac{3x}{4}+\frac{6x}{4}$3x4+6x4

Think: Now we have a common denominator, we write the fraction as a single expression over the common denominator and then simplify where we can. 

Do

$\frac{3x}{4}+\frac{6x}{4}$3x4+6x4  $=$= $\frac{3x+6x}{4}$3x+6x4
  $=$= $\frac{9x}{4}$9x4

 

Reflect: We collected the like terms of $3x$3x and $6x$6x.  Are there any other common terms?  Are they any common factors with the $9x$9x and $4$4?  No, so this is a simplified as this answer gets. 

 

Practice Questions

Question 1

Simplify the following:

$\frac{6x}{2}-\frac{7x}{2}$6x27x2

Question 2

Simplify the following: $\frac{3x}{5}-\frac{x}{7}$3x5x7

Question 3

Simplify the expression $\frac{11x}{14}+\frac{7x}{21}$11x14+7x21.

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