Step by step
The question
Question: find f(1/3) for f(x) = 3x − 4.
f(x) = 3x − 4
f(x) is read "f of x".
It is a rule, called a function.
A number goes in for x.
The rule gives one answer out.
f(x) = "f of x"
x goes in → an answer comes out
1 Put the number in
f(1/3) means: put 1/3 in place of every x.
We write it in brackets.
f(13) = 3(13) − 4
2 Times and divide
Times comes before minus.
Order: ( ) → x² √ → × ÷ → + −
To multiply fractions, we do top times top, and bottom times bottom.
A whole number is a fraction over 1.
= 3 × 11 × 3 − 4
3 times 1 is 3.
1 times 3 is 3.
= 33 − 4
3/3 means 3 divided by 3.
That is 1.
= 1 − 4
3 Plus and minus
We take away more than we have.
Start at 1.
Go down 4.
We pass 0 and land on −3.
f(13) = −3
So f(1/3) = −3.
Let's look back.
First: put the number in.
Then: times and divide.
Last: plus and minus.
Answer
f(13) = −3