# Math Expressions. How to write simple mathematical expressions.

When writing down expressions in algebra you need to write them down in a specific way. In algebra a letter can be used to stand for an unknown quantity. Adding and taking away are straightforward:

x+7 means x add 7

x -7 means x take away 7

but times and divide can be harder to do:

7x mean 7 times x

x/7 means x ÷ 7

Let’s have a look at a real life example.

**Example 1**

A fish pod contains a large amount of fish. Write down an expression if:

a) 1 more fish is added to the pond

b) 2 fish are taken out of the pond

c) 3 times the original amount of fish are added to the pond

d) the amount of fish in the pond is divided by 4

Answers:

First of all use the letter f to stand for the amount of fish originally in the pond.

a) f + 1 since there are no special rules for adding.

b) f – 2 since there are no special rules for taking away.

c) 3f since if you are multiplying the number comes first and the letter comes second.

d) f/4 since you always write a division as a fraction (with a line).

**Example 2**

A line has a length of y cm. Write down an expression if the length of the line is trebled.

Answer

Here we are multiplying the length of the line by 3.

So y×3 times by 3 can be written as 3y.

**Example 3**

Another line has a length of z cm. Write down an expression if the length of the line is halved.

Answer

Halving is the same as dividing by 2.

so Z ÷divided by 2 can be written as z/2.

Top Tips:

Just remember when you are multiplying put the number first followed by the letter, and leave out the multiply sign.

Use a line for dividing.

## More by this Author

- 0
If y is directly proportional to x, then the formula connecting y and x is y = kx. Direct proportion means as x increases then so does y. k is known as the constant of proportionality and this is found by...

- 4
In this hub I will explain the difference between inverse proportion and direct proportion. If m is directly proportional to n then the formula is: m = kn If m is inversely proportional to n then the formula...

- 0
The density, mass and volume triangle is as follows: So if you wanted to work out the density, you would cover up density in the magic triangle to give: Density = Mass/Volume (since mass is above volume) So if...